Being a highly sexual woman is a blessing and a curse, believe me.
Your position is especially challenging because it's just not the norm presented in society. Women who want sex more than their partners seem abnormal; like it's wrong for you to have the desires you have.
But listen to me: A zest for sex and sexual expression is PERFECTLY NORMAL! This is nothing to feel ashamed or embarrassed about. I know it's easier said than done, but the more sexual women you surround yourself with, the more normal and comfortable it feels. Hang out with people who get what you're about.
Sex is about as vulnerable a thing as there is. You're naked, you're putting yourself out there... and to be rejected when you're in that position will really mess with your self-esteem. Facing that exhausting battle, over and over again, can and will wear you down. It sucks to feel unwanted.
I think it was pretty noble and selfless that your boyfriend let you get what you needed elsewhere. Sometimes, it really is the only answer when your partner isn't able to satisfy you and doesn't know what else to do. That being said, I can understand why it only happened twice. There is a big difference between having a real open relationship because the two of you want to explore other avenues of sexuality, and letting your partner have sex with another guy because you feel so inadequate.