I think I can see the psychology here. I reckon that if this girl wasn't the ex of your current love interest and was a friend, acquaintance or even just someone you knew, these intense feelings of jealousy and inferiority would be non-existent or very minor. She once had what you want now - a proper, serious relationship with this guy - and you are still in the early stages of things with him and are unsure as to whether you will make it to that next stage. It's natural, when seeing starting to see someone we really like, to doubt ourselves and wonder whether we measure up to their 'standards'. But too much and it becomes unhealthy, as you have acknowledged.
Your brain, unfortunately, is thinking 'X (the girl) got Y (this guy) and X has the following traits A, B and, C, and so this means that to get Y I must have traits A, B and C'. This is not true. You do not have to be a carbon copy of someone's ex for them to like you. Men are capable of liking different things in different people. Look at all the people you have been attracted to - have they all been similar? Doubt it. This sounds a little odd but in a way you are seeing this girl through the eyes of this guy, and the only reason her 'greatness' seems so 'great' is because she has been 'validated' by him.
It's interesting to look at the way you have described this girl. Take some time to try and describe yourself in only a positive way. There are different types of attractiveness and just because you don't fit one 'stereotype' of 'beautiful' doesn't mean you're not beautiful.
As someone else said, if you can't get past this, move on from him. Your health is more important.