First a BIG Grandgpa HUUG....then my thoughts
You state that you are 21, and by that you are on the door step to adulthood.
I think the best benefit you can have, is to work with yourself to increase your self confidence, and understand your own worth.
Your story is not unique, it happen all the time, even if we (and I am sure if you read your own post as an outsider) who see it from outside can't understand why a person "downgrade" oneself so much as this.
It is not said in any Negative way, but I think the urge to get something like this to work, is the "scary thing" to be left alone, to the unknown.
If our self confidence and trust is increased, then we will not tolerate such an behaviour.
You can never change any other being then yourself, it must be a will to change, and you two have tried more then enough to understand that this is not the right partner.
When a relationship develop like this, in the early stage, then how will it be later, is this a partner and a situation you will grow old with?
The same goes for your girl friend, why do it hurt you that she is together with him?.....you have seen how it works with you....so the hurt must more be a hurt self image that they might get something function that you did not.
Again will self development, increase your strength, and maybe make you understand that we all are different, and since we are different we can make different thing to function.....now let me at once state that I don't think this relationship will last either, for me it seems the guy is on exploring, jumping from flower to flower, and maybe he one day will find the one he will settle down with, but don't let it be YOU!
Let him go, you are still very young, let life unfold, be happy and positive, and that will attract similar people, and you might soon spot your prince.
Life are as a beautiful rose, when everything goes our way, then the flower blossom and we can smell and adore it, when life is cloudy

we might grab around the stalk in order to fetch something, and that might hurt us by the stalk's thorns.......but the flower and the smell are still there....just take another approach and understand it all is about learning.....what a dull life it would be if everything was a success....you would never know you had it...because you never have discovered the opposite....so make yourself strong....and life will more easy smile to you.
Good luck
