Why am I the in between girl?

Sophia
Let me defined you the word" in between girl" I've encountered many guys who just "not ready to be in a relationship or I need a break" but comfortable enough to see me naked. Then after a few months of fooling around they end up in a relationship with a new girl. And I ended up crying in my pillow wondering what could had happened? 
At first because I though I let all the guys uses me too much with hooking up and let them take advantage of me whenever but my recent guy I tried to not make the same mistake, I take things nice and slow with him and try to get to know him, until now it been 4-5 months of talking to him and going on a date but he has not yet make me his one and only and last night I saw him, flirted with another girl and asked for her number when I accused him of doing it he just apologized and told me it wasn't serious. I got really upset with him and don't understand if there's anything wrong with me? I can't help it but to think it was my fault, I been with around 8-9 guys my whole life and I toke thing serious with them and each one I tried to fix the mistake but until now I'm always the in between girl who no one want to take thing serious with me and i stucked with the line and wondering"what if?"