Why this keep happening to me?

TW: a comment from a creep with a large age gap

Looking back, I realize that I am bad at recognizing what true friendship really is.

My first best friend (we have been together for 20 years) stole my pencil upon our first meeting (we were 7 years old that time). I cried and she got scolded by the teacher. We did become best friends, but she never defended me from bullies, always discarded my emotional plight, and was jealous of me for "stealing" her friends because I was the smarter one.

I also had another friend whom I was close with. But looking back now, I realise that he's a creep because he said that he wished he could marry me despite he was already married that time, and to make matters worse, I was 18 that time, but he's almost 30 that time.

I lost a friend to a huge argument almost two months ago, and I was really upset at myself. But looking back now, I remember that our first meeting had her getting offended by something innocent I had said, and we had a short fight in the public chat server, and I was made to feel bad about what I just said (despite nobody else was offended by it). Not to mention that there are times she was being hypocritical (but I never called her out because I didn't think much about it), and she just gave toxic positivity vibes.

I am just scared that I can't protect myself from people who want to take advantage of me... Because I have gone through enough...