I know it sounds odd, but you're kinda supposed to mess up at drawing. Or at least that's how it feels to me in my experience.
Remember Bob Ross, the whole "happy little accidents" line he liked to say? He didnt just say that sort of thing just to be calming and whatnot. He said it because it was important to understand.
Like, some of the stuff I've shown on here (that is on paper, not the digital stuff). Most of those have some big mistake on them, I cant get through a drawing session without that. And often these happen with tools that cant be erased, like my markers. So, what to do? Improvise. Change up the drawing so that the "mistake" now blends in as part of it. It's no longer a mistake then.
Seriously, happens to me every darned time.
Though also, art requires a lot of practice. Sketch pads, you're kinda expected to go through some of them creating things that arent exactly meant to show to others. Stuff like line practice and whatnot.
Here, look at this:
I have pages and pages and pages of stuff like this. This is my lettering practice stuff. This specific pad is made FOR that but I have another marker pad that also is probably half of it like this and then the other half is being used for actual finished stuff.
If you're practicing, which you totally should be, you're going to have pages and pages of stuff like this too. I mean okay not EXACTLY like this, but you get the idea. Maybe you'll have lots of pages of character poses or even just shape & shading practice, or whatever.
It's just part of the hobby, sir.
I suggest that if you start on a drawing and you immediately like REALLY screw something up, take that page and make it into a practice page instead. I've done that one, where I like start something and sneeze and there goes the freaking marker across the page, and then... yeah, that becomes a line & curve practice sheet.
Anyway, I had been stuck in the way you're thinking for the longest darned time, and then somewhat recently it's like, you know what... heck with it. Just gonna make stuff. If it comes out good, cool. If it doesnt, hey, maybe it'll atleast be funny. Once I stopped worrying about it so much my output got more consistent and more creative.
Also dagnabit stop with the age thing. Of course you can keep learning and improving.