It seems like you have very low self-esteem. I don't think what you are really seeking is validation in your beauty, but rather, some reassurance that you are still loved and wanted in your relationship. You have value and you know it, and it causes you to be distressed when you don't feel you are being appreciated, otherwise you would not have worded your question the way you did. You do not want to know that you have value, but that you are valued - that you are desired, treasured, and loved. There is nothing wrong with that, and do not let anyone - even your boyfriend - convince you of anything else.
Cultivating a relationship and maintaining it is hard. A lot of people are under the impression that just being together and staying together is enough. It's not. You seem to put quite a bit of thought into your relationship, how to forgive, how to move on, how to improve and be desirable. It is evident that you love the person you are with very much. Sometimes, this is not enough. When you love some one, you see past their faults, you accept them as they are (because honestly, change does not come easy to most, if you are going into a relationship expecting change, you are bound for disappointment), you put their happiness first, and while forgiveness may not come easily, it is something you always aim for because you love them and it's worth the struggle. That is a lot of what a long term relationship is forgiveness, redemption, and compromise. You have done much of this. You have forgiven him for his online affairs and his flirtatious behaviors, you excuse him for neglecting you by trying to take responsibility for your own happiness, you have started changing who you are for him to try to meet his desires. You have said goodbye to people in your life who have challenged your relationship, and still, what sacrifices has he made for you?
If you have to make yourself feel beautiful, valued, and happy because of his indifference towards you, then there is definitely something amiss, because healthy relationships do not function that way. This boy says he wants to marry you some day. This is not a decision that should be made lightly. A relationship cannot thrive when only one person is making sacrifices. It leads to resentment on the behalf of the person making sacrifices and it causes the person relying on those sacrifices to take the other for granted.
You will depend on this boy even more in marriage than you do right now. Marriage changes things. You become more cut off from the world as your efforts are put into building a family of your own. Your paths become one as you aim for a future together. Ask your friends if they've noticed changes in you. Have your life goals suddenly changed since being with him? Would you want him to treat your future children together the way he is treating you? Consider these questions before marrying, please.
No one would fault you for saying goodbye to some one unhealthy for you.
Keep hope Desirae.