How can I learn to trust again?

Hello everyone, and thank you for reading my problem... I'll keep it as short as possible. Some things happened in my past that made me promise myself I wouldn't trust my feelings with anyone, that I would never get close to anyone ever again. I just didn't want to deal with that kind of emotional pain again, which is very tiring and, to be honest, I'm not a very strong person. I'm not saying I don't have friends, I have lots of friends. However, only two of them are actually really close friends who I really care about. As you can imagine, whenever I've had any kind of chance of starting a relationship I back off because I don't want to risk getting hurt. Last year I met a girl that was somehow able to see through me. I got very, very close with her and my heart decided it was time to let someone in for once in more than 10 years. Long story short, she hurt me even worse than I've ever been hurt. The thing is, I'm tired of feeling lonely and being alone (in terms of relationships), but I just don't feel like putting my guard down ever again. TL;DR: I got very hurt when I was very young by people I cared about, so I decided not to let anyone else in. Finally, after more than 10 years I meet someone who I completely trust, I open up and let her in and I get crushed worse than before. How can I learn to trust again and open myself up again?