Things in life that happen can change you for the worse or for the better. We are wired for survival which is great in order to survive, but it also means the bad stuff can have a larger impact than the good stuff.
Think of three times someone maintained your trust. Now think of one time someone really broke your trust or betrayed you. The bad stuff is remembered faster because in survival, a fast reaction is needed.
You can move through the bad things but it takes making a choice, conscious effort, and time. You can falsely think that you "learned" not to trust anyone and that will keep you safe, but it won't. It will imprison you in a world of mistrust and prevent you from experiencing deeper connections in life.
I'm not saying you should go, "Hi, nice to meet you, here's all my bank account information and all my darkest secrets." But trust new people the same way you want to be trusted.
You don't want to be punished for what someone else did so don't punish people for what others did. It will only push people away and hurt them.
Trust unless or until trust is broken. It's only fair.