welcome to pc. i've experienced trust issues in the past. what i found was i was too willing to place trust in a relationship of any kind without using good judgement. that caused relationship and trust issues for me. it takes time to build trust. even with loved ones.
i wonder if something in your past skewed your ability to trust thus keeping ppl at a distance? this happened to me. you can learn how to balance this out but only if you are cautious of warning signs in a person. if you see warning signs, them talking about someone else in a gossipy way, etc. then back off being an open book. it doesn't always work to perfection. your bf is an example of violating your relationship in a big way. good luck with your re-building trust. it won't be easy. i'm sorry that happened to you.
therapy helped me to learn how to differentiate ppl who are trustworthy themselves vs. ppl who aren't. i couldn't do that alone. was clueless at first.