Part of not being able to trust others is the belief that you are somehow unworthy of happiness.
And, by extension, you don’t deserve to be treated well.
Low self-esteem and self-worth often come about for the very same reasons as the trust issues.
People treat you poorly and this degrades your ability to trust and makes you believe that you must have deserved it.
And remember, trust involves someone else taking into account your feelings, wishes, and best interests. But if you don’t believe that you are worthy of fair treatment, why would you trust others to consider any of those things?