Chances are, they won't be able to tell if you have your guard up or down as you sound like a pretty reserved person!

I have a loooot of trust issues. I tend to befriend people on the assumption that at some point, they're going to bail out.

I don't share too much of importance with people - and I start out gradually and only share bits at a time really. It can be really hard. I'll withdraw from it quickly if I think that they aren't as trustworthy as I thought... and I rarely give a second chance in that regard.

Do you go to see a T? They would be able to work with you to try to work through some of this!

I wouldn't promote, trusting blindly. I have trust issues, myself, not quite the same way you've described, but I certainly can be a bit cynical.

You can self-teach yourself, about body language, word usage, etc. That could be one step, you could take to get to the point of learning how to be vulnerable, around others. Who to trust, who not to trust, can come from retraining yourself. Take that moment, of doubt, ask yourself where it comes from, and whether it's the person, the situation, or your past that is creating this type of mistrust response.
It takes time to heal from betrayal, which you expressed happened with one of your close friends.
I know there are on-line articles, just not certain of there are any books on the topic.