Hi Ruby, I'm going to echo a lot of what others have said but having been both a (now acknowledged) stalker and a frequent stalkee, I have a perspective from both experiences that may enlighten you a little as to how the other person feels. You are young also, most young people have a hard time not being clingy and obsessive, time and experience will improve this for you but your sister and the posters on the forum are trying to save you a lot of pain, many probably from experience with this on one side or the other.
It sounds to me like you are infatuated with her looks but maybe not really who she is as a person. As someone once (or several times...) in your shoes, I can say the idea of getting to know the person and becoming disappointed once I did so because they were not, in fact, the ideal I'd held them up to, was terrifying. In one case I only ever managed to say hi to her (or hey, I forget which I decided was ultimately creepier). In another, things went really terribly for me when I decided to act on my feelings.
Situation A: I revealed my feelings. At first, the guy liked the mysteriousness of the flowers that suddenly appeared in or outside his dorm room (he left it open). I didn't do this for long enough to be truly creepy but it really could have been. I eventually outted myself as that person, which is how I know he had liked the surprise. We made out, that was pretty much it. I went to a show where he played guitar and sang and took a video of it, which he later requested I send to him. None of his friends sat near me. He invited me to a friend's house, I recall being severely disliked and feeling awful about myself, but I'm not sure what went down there. He had been drinking so he wanted me to be the designated driver back (probably only reason I was invited), I got lost driving because I wasn't familiar with the area and get panic attacks about that sort of thing. My obsession continued, I looked at the video of him all the time, pictures on Myspace (then a thing), and posted awful 18 year old "love" poems on there, of course, about him. I found out through the college grape vine he and his friends were making fun of me incessantly, really cruel things I will never recover from. I found out from a mutual friend of ours who had stood up for me and actually his ex-girlfriend as well (she messaged me that she thought I was sweet but that these people were saying really horrible things and she had told him what he was doing was wrong). The humiliation led me into a lot of mistakes trying to run away from feeling anything for anyone anymore. Aside from the Myspace posts, I actually don't think I did THAT much wrong in this situation but it was enough to crush me completely. I didn't know really anything about this guy and, if I had, I wouldn't have liked him at all.
Situation B: I was stalked by a fellow aspie. I've been stalked by others who scared me more than this girl--80 year old men, men who attempted to abduct me when I was 16. I felt completely helpless when this girl began stalking me because I knew exactly what she was doing and knew the only thing that had made ME stop had left me shattered. I didn't want to hurt her like I was hurt, but I knew it was impossible not to hurt her in some way to avoid that. It was a lose-lose situation. I met her at my first drag show, I was 20 or 21 and kinging to a Bloodhound Gang song. It was very scary for me to be on stage but conquering that fear did a lot for my self-esteem at the time. She was in the crowd and had given me a dollar, so I paid a little special attention to her, as I did anyone who gave money. After the show, she bombarded me with heart-shaped eyes, begging for my phone number. I told her I didn't give my number out, and that was true. I offered she look me up on facebook instead, which was a bad alternative. I'm not sure if she liked male me (admittedly, I make a hot dude) or female me, because she had previously been straight. As I learned later, she came out as bi after this and her sister (a lesbian) soon after had become one of my best friends and she would later also have romantic interest in me (difference was, she knew me very well). This made me feel uncomfortable because I did not want to hurt to sister (the aspie), who also liked me and would be crushed if I dated her. My stalker messaged me regularly declaring her feelings, I turned her down very kindly but she persisted. She had her sister (then unknown to me) drive to a Gay Pride Parade abruptly saying she had "plans to meet a friend" when I'd waited until minutes before leaving to post a status about going, had never conversed with her about it. She followed my friends and my own crush-obsession of the time around looking for me, and I think it was the following my crush (who did not know me) and asking my crush questions about me that infuriated me because I finally came out of hiding and, when she grabbed my shoulder, I snapped at her to leave me alone. She did, but I felt horrible about it. She called my friend whose number was on facebook to ask about me, she had no idea who the girl was. This prompted me to send her a firm message that I would need to remove her as a friend and request she not contact me. I did not put her down or anything, but I knew I had to be very firm to get the point across. She left me alone. I later became friends with her sister, who told me she had suddenly turned super-Christian, got a boyfriend and a tattoo of Jesus, and even got a little gay-bashy. I think she was experimenting with identity, had looked up to her sister a lot, probably found me attractive and clung to that.
I felt really uncomfortable that she had gone through and "liked" pictures of me, even pictures not tagged of me and not on albums of anyone she would have any access to as they were relatives and friends from my hometown. It was really creepy feeling like nowhere was safe from her eyes.
Situation C: I have also dated a guy who was obsessed with me. He wasn't interested in getting to know me as a person at all and had stopped doing the thing that had originally caught my interest in him: his art. All he wanted to do was be around me. He got mad that I worked so much, and part of why I did was to get away from him and have some time to myself. He became extremely controlling and did not want me to go to a coed college because he felt extremely threatened by men, but since he knew I liked women, he also didn't trust me around my attractive female friends and lost it when I put my head in the lap of one. He accused me of cheating on him because prior to dating him I had messed around with someone else. When I broke up with him, he told me if I left him he would stab himself in the hand, which he did, in front of me. Later a barrage of insults were thrown my way by his supporters, people who didn't understand why I was leaving him, we were so perfect together. He orchestrated a plan to try to "win me back" by making me jealous and dating my friend, who had called to ask me if it was okay, I had told her it was but that I cautioned her to be wary of getting too close to him for her own safety. I realized later he took no interest in anything I liked or did, in fact he pushed me away from those things. He really only liked how I looked from afar, never thinking I'd become his friend or girlfriend and once he had me, I think it made him overwhelmingly insecure.
Anyway, I really feel for you, this does need to stop though to protect yourself and your romantic interests. I implore you to get to know her as a person. That way, if you like her, you like HER, not what you think she is like. Ride out the intense feelings, try to find more creative outlets for them. Put your energy into yourself. I mean it, obsession can DESTROY you emotionally and even socially. If counseling helps, try it; I think you can also help yourself by finding other interests to occupy yourself with. I don't think your obsession itself is not NORMAL, but some of the ways you are expressing it certainly are abnormal and unhealthy. Try not to focus on the feelings of being rejected or lectured and really listen to the compassion in the advice.
*butterflies*
