Think of something you like to do. Something fun: riding roller coasters, going to see your favorite band perform, or going water skiing for example.
Then think about your partner not being with you when you do whatever thing you're thinking about. Are you still going to have fun without him?
Are you going to have fun because he isn't with you or are you going to have fun simply because you find the activity fun?
Think of things your partner does that he finds fun. Is it only fun when you are not along with him? If he goes and does something he thinks is fun without you - are you jealous?
I don't just enjoy things if my partner participates. I enjoyed sex before him so clearly my enjoyment of sex isn't dependent on him being my partner. And if he enjoys sex with someone else it isn't BECAUSE it isn't me he is having sex with. Its because sex is fun. Its not as personal as you're letting it be when you are feeling jealous.
So when you're feeling low about it, remove yourself from the equation and then look at what he is doing again. Your partner isn't seeing others because you're not good enough. Its because getting to know people is interesting and sex is fun.