I've never been able to trust anyone, not even my friends so as you can possibly tell i don't have many friends because I am always holding my guard up. I used to think that if they were on the phone, that they were messaging my boyfriend. Id check whats app and see when they were last online and then go check my boyfriends...I done loads of stuff that in my mind led me to think that I was being played by everyone and trust me it is hard, im still going through trust issues now with my boyfriend but find talking about things to him helps (sometimes)
I've seen a counselor who helped me understand that the reason i don't trust No Man is maybe because of what happened to me as a child. I dont know..

Some people, do have a natural gift/inclination to do better at it, than others. But it's something, that even with this, one learns through secondary education courses, or self-directed, or honed through life experiences.

Understanding Body Language - Reading Body Language
How To Read Others' Body Language In 3 Simple Steps - Forbes
https://www.google.com/#bav=on.2,or....ed=0CAoQ_AUoBA

The above links, are just some general articles and one is a link that has pages of books listed, that are all about this topic. Hope that helps.

First bit.. why would you think that almost everyone is hell-bent on hurting you? Without sounding mean whatsoever, but it's something my fella's therapist said over his fear of people looking at him - but what makes you so special? By special, I mean the person that nearly everyone wants to hurt? People are not like this. Yep, there are a few tossers out there, but by my experience, people don't go out to hurt people, especially physical. I wonder why you think that everyone is out to get at you.

Second bit.. a little of what I just wrote applies here. People do not generally wait until your guard is lowered to then get at you. Seriously, unless you've very very unfortunate to meet a really bad person, or you've treated them bad and they want revenge, then people just do not do this kind of thing. They are too busy living their own lives for one, and for another, people, generally are not this cruel. People DO make mistakes, and are capable of hurting others sometimes by being uncaring or selfish, but they are rarely this cruel.

The third bit.. about the gun. I can't think of anything rational to say to that because you're thinking isn't rational. I think with your replies here, you do have a trust issue, but I also think that you are somewhat paranoid. You say that therapy doesn't work, but I really do think that you need some. You say that you clam up and do not trust a therapist, but yet you are here asking how to learn to trust. I think therapy, and really really go out of your comfort zone, really try your very hardest. Push yourself to talk to them, learn some tips from them with this trust and paranoia, learn to cope better with hurt. I am sorry you feel this way and it must be incredibly hard to live a life like this. I hope you find some help soon, even though it will be very hard work on your behalf. Better than living scared of your own shadow though?
Massive hugs.