You may be able to be friends with your boyfriend at some point, but not now. Breaking up usually works best when the break is clean. By breaking up with him because he’s going away, but then agreeing to stay together until he leaves didn’t really make it a clean break. It must have been hard for you because you broke up with him because he’s going away — not because he cheated on you or abused you or did something wrong. You broke up with him for circumstantial reasons. Perfectly understandable and reasonable. The problem was when you flip flopped. It probably seemed like a compromise to you, but to him, it was a way to keep his options open.
When he met someone else in Boston, he pretty much had a license to move on with his life since your getting back together was conditional on being together only for the weeks before he left for his four month trip. If you hadn’t agreed to stay together until he left for Europe and had made it a clean break, it wouldn’t have hurt so much, because he wouldn’t have betrayed you, but it would still have hurt because he moved on so quickly after your break up.
Break ups are painful, it’s always good to see someone’s true colors because you can conduct your own life accordingly. Would you really be able to trust him if he came back to you now, for the few weeks before he goes away for the summer? I think not.
Focus on your own summer plans and yourself. Breaking up is hard to do, but when one door closes another opens — and I bet in your case more than one door will open since you’re newly single. For now, don’t try to be friends with your ex-boyfriend. Let the healing begin, move on, and see how you feel about a friendship with him after six months or so.