Same problem for me, it's a big part why I try to stay as isolated and protected as possible from other people - unless they've prooved to be reliable for a long time.
What helps me is to constantly separate my feelings and how I feel with what my reason and logic says about a situation. That way I'm less influencable, but it's a constant mental work. I cannot trust myself when it comes to being influencated by other people. What I can trust is my intuition when one side of my mind says "not good". In general, this side of my mind never betrayed me althrough it's "dark" the message is pretty clear, while the influencable side did me no good, ever.
Also, realizing that I've got the best intentions for myself while others might not have good intentions and/or might have no idea about what's the best for me was a big step.
I know that my own judgement about any type of situation is wanting me 100% good, while the others might not. I rarely was wrong even if I'm hesitant to trust my own intuition. I regreted a lot more in my life that I followed other people's influence on me and advices and opinions and viewpoints than my own. I rarely told myself "if only I had listened to this person", but it's a repetitive thing "if only I has listened to my own impression and not what that person was saying, ****". Now, I rather be wrong because of myself than be wrong because of following others. If I'm wrong, I'll assume and take responsability. No one takes responsability for me, therefore, no one is allowed to play games on me.
So, separating my feelings and emotions with a person VS what my reason and logic and sense of what is right/wrong say is very helpful. Having alexithymia doesn't help, because I might need some time off to figure things out; it's really difficult to do that spontaneously for me. But what really changed is realizing that I do have the best intentions for myself, therefore, a lot of my judgements are actually the best for me; even if they imply to be negative about others. I'm not blind. Given your message, you're not blind either, just unsure. I don't know if you're like me, but I know I'm unsure of other people's intentions because althrough I see that they're manipulating, I can't read their intentions CLEARLY. I can't read faces well, I can't read expressions very well either, I also miss a lot of subtext and understand things literally (etc), which is something that makes me unsure about being right or wrong about them.
I solved the problem in my mind by chosing my own safety and protection and follow my judgement even if it's not 100% sure. If I was wrong and it matters, then I'll change my mind without problem. I prefer being proven wrong but having kept myself safe and be more responsible for me; than being proven wrong by having taken risks and let myself be harmed "because I wasn't sure". It's a better strategy for me.
Also, I'm totally paranoid in contact of people being nice and warm. "Why are they doing so? What are they interested in?" is my first reaction. I'll check that until the deepest hidden parts. If there's something I'm suspicious about, the person is out. Again, I prefer being proven wrong in a good sense than in a bad one. Because being proven wrong in the bad sense implies that it's always me paying for other people's ****.