I don't trust my boyfriend

I found messages between my boyfriend and this girl on social media. He was asking her for naked pictures and then hung out with her once, but nothing happened. I didn't find out about the messages until last month and that was six months after it happened. I can't trust him anymore and I'm just wondering if I ever will. He admitted that he wanted to have sex with her but he didn't. He was only talking with her because he thought we were going to have a break up. My heart is completely broken. I feel like I'm in competition with every girl on the planet for my boyfriend's affection. I'm afraid that he doesn't like me anymore, but without me he's homeless and without a car. I love him so much. It's been tearing me apart. I don't feel pretty anymore though. I don't feel good enough for anyone or anything. I thought I knew him as well as I knew myself. Then all of a sudden my world was flipped upside down. I'm still trying to figure out which way is up.