How to regain trust for people?

Here is this guy who I saw for a while , He told me he wanted a relationship and acted like a boyfriend material besides that one time that he made out with a girl in front of me , I forgave him because he said this girl really liked him and used him when he was drunk. The main reason I stopped seeing him is that I just knew he has a long distance girlfriend, that's when I realized I was used and manipulated into having sex bc of how much I want a relationship with him
I left but I realize I can't trust people any more? If you had a similar experience , how do you regain trust for people? How do you avoid this situation if you never experienced it?
How to regain trust for people?
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