Wow. WOW. Reading some of the things that have happened to you sound so familiar.
I have a lot of advice and here it goes.
First. Try and think of the fact that this is happening before you got married and had children as a benefit to you. If there ever was a time, unfortunately, this is the best time for him to be doing this. God willing, if everything works out, this will be better for your relationship leading to marital committment.
2nd: Take a deep breath and let it go. Let it all go. Let him go and do "what he feels he needs to do." You don't want a man who doesn't want to be with you, right? I truly believe in the "I love you so I'm letting you go."
What are you doing for yourself now? And, something I learned, what are you doing for yourself for the improvement of yourself, rather with an alterior motive of helping your relationship?
Truly--I want you to know when I was reading your post I could understand what was going on with your boyfriend. You both have been so focused on yourselves and your education. Then buying a house! That is what did it for my husband and I. It is like the symbol of "becoming an adult". And your boyfriend has spent most of his 20s getting his education, when has he had the time for independence? It is not right he cheated on you. NEVER and I still cry over the betrayal from my husband (did just two nights ago). I'm an optimist, though. I would like you to look at this and be grateful that you are getting this time to reassess what YOU really need in a man, a husband, a partner. What YOU really need for YOU.
Good Luck and we'll talk more soon.
H & I, both 32, together since 18. *M 7/03, A since 9/06. Bomb 7/07, H ended it w/ OW 9/08 * Agreed to D 6/09...very hard *D 8/10 * At peace, have become great friends w/ X-H and his new GF