So...when I'm having sex with my boyfriend I secretly think of him having sex with another woman, sometimes in this 'fantasy' we have a threesome with her. But mostly it's like he's doing to the woman what he's doing to me at that moment. This is the only way that I get turned on, let myself go and climax. This 'other woman' changes a little every time, she isn't the same person but always has the same kind of features, it isn't someone I or my boyfriend know or have ever seen. I just think of a woman that I find beautiful with features that I don't have, she doesn't resemble me at all. Someone I can never be. 

In this fantasy he is always reaaaally into her and very turned on just because of how hot she is. In the threesome variation of the fantasy I'm basically non existent, it's all about her and how attractive my boyfriend thinks SHE is. 
When I think of him having sex with me at that moment, it does nothing for me. I don't want to think of another woman but it seems like I need it to even enjoy sex with him. 

This is weird to me because I'm an extremely jealous and insecure person. Just the thought of him having had sex with other girls in the past, before we were together, makes me miserable and angry. So the thought of him having sex with another while we're together or in the future fucks me up even more. I would never have a threesome with him either.

And still, during sex it's the only thing that gets me going. It really bothers me and makes things hard because of this I get even more jealous or insecure and convinced he'll leave me for another woman. A kind of woman that's more like the one I think of (he has girls like that at his work, is related to them, I seem them when we're out etc.). 
Girls who look like that are also often interested in him. I feel extremely threatened by them and get really angry and crazy when I notice them around my boyfriend. I don't know if he finds that sort of girl attractive (I don't want to know 'cause it would drive me crazy). But I think a lot of people would, so in my mind he'd be weird not to. I also have this conviction that he's out of my league and he'd eventually end up with a girl like that. Like I'm just an ugly or mediocre looking loser and he actually doesn't want me. I mean, girls like that exist and they want him?! 

I was wondering if this whole thing (thinking of him having sex with this kind of woman to get myself turned on) might be due to low self esteem? Is it because I don't think I'm beautiful and desirable and thus think he couldn't be turned on by me either? 
I'm wondering what people think of this and maybe even if someone can relate 