Why did I do it?

So Monday evening, my fwb and I had sex. I know that relationship is only ever going to be that. I have been texting this other guy, his car broke down so I picked him up. He came over, showered, and we ended up having sex. I feel bad because I keep giving up myself to people who aren't worthy. I don't know what to do right now. I have been crying all night because I feel so used.