Peanuts - It is no surprise that this is seriously affecting your self-esteem. That you feel 'the better the devil you know' - there can be life without a devil at all, you know! Also, trying to be 'realistic' that he would be unfaithful - actually, although no-ne could say infidelity was exactly rare, MOST men are not!
But you are with a man who treats women like objects to be bought. The obvious implication of this is that he gets you for free. He does not see sex with women as part of a loving mutually respectful relationship, but as a transaction, at his beck and call. So, by extending his feelings about women to you, he is treating you like an unpaid prostitute.
The longer this goes on the more you will feel that you don't deserve better.And you DO deserve better - every woman does. You have not CHOSEN to be treated like this (unlike maybe some of the women he buys). Also, personally I would be filled with disgust at a man who can buy sex with women some of whom may habe been trafficked, be forced by a pimp, etc. This is not a case of the old 'tart with a heart being kind to a lonely man with needs'!
However, your self esteem has aleady taken a bashing. Can you get counselling, or find an assertiveness training course? I think what he is doing to you is emotional abuse, and if you call Women's Aid (it isn't just for women who get bashed, you know!) they may be able to direct you to courses, groups or counselling that will helpyou build up your strength.
Otherwise, if you feel up to it, and have enough RL support, go to the CAB and discuss all the aspects of leaving him or throwing him out.
Good luck - you sound lovely, and you are young. Don't write off your life now to this man.