I really need help, I think

I can't seem to get over the fact that my boyfriend (who is older than me) has been with other women. It kills me whenever I think about it. It's not a matter of comparing our pasts - I'm a lot more inexperienced than he is but that in itself doesn't bother me. Just.. imagining him with other women, having been intimate with them, wanting them, having that sort of history with them.. Not to mention he's been married. Having had that kind of.. intimacy before with someone who isn't me, I can't stand the thought of it. I torture myself thinking about it to make myself feel bad.

A lot of this crops up at weird moments, and I don't really talk about it. I have expressed this insecurity before though, and he thinks I will grow more comfortable with it with time. He has also reassured me many times, but I know he can't do more than that. It's my issue. And I know it's a matter of perspective, I just can't seem to shift it in a way that makes this sort of thing not matter anymore..

Does anyone feel the same way? If it stopped.. what helped?