I can relate to your issue very well....I can understand your pain....I am the same like you....and that's why I'm always ending up alone....
with my ex-boyfriend, I was getting jealous and suspicious all the time...and good lord, he had lots of girlfriends....and making more....I was getting jealous of him hanging out with his step-sister....I wanted him all to myself....but that wasn't possible....he told me that I'm insecure....and he was right....I was like you not feeling that I'm good enough for him....
anyway, he ended up....and now he's with another girl...that girl doesn't care...she shouldn't be a jealous and insecure then....and now, he posted in his facebook that he's in a relationship with her....he moved on quickly and teared up my heart....It was so painful seeing him coming to the dance class with the girl...I felt a sharp knife in my heart....but now that time is passing, I'm getting better....I try to talk to other guys and let them to cherish me....if even I don't like them but it's good to get attention....and be desirable....I signed up for online dating and it's fun....I haven't met anybody yet, but got connected with some guys....and there is hope out there....just you got to be paitent...and when that right person is in front of you who is giving you all the attention then you don't feel insecure and jealous....trust me....I've been there and I know how it feels....
with one of my boyfriends that I felt so much love, even when we got separated and he got a girlfriend and I was single at that time....I didn't get jealous even seeing them together.....because I knew how much he loved me and just because we couldn't be together, he had to go with somebody else.....so, if the one that loves you comes along, you won't feel this way.....
just let time heal your wounds.....
take care
Marjan