Why did your boyfriend change his behavior toward you? I have no idea. You two have been through a lot together. You used to talk honestly with each other. And that's the most important thing you two need to do now. Assuming you're both willing to do that, it sounds like the first agenda is to describe exactly what relationship each of you wants. Then you need to see if there's enough overlap between your two visions to create a relationship you both want. If not, it's time to move on.
If so, I hope you don't blame non-monogamy for your broken heart. Your story illustrates both some advantages and some disadvantages of open relationships. To date, it appears as if open relationships are as complicated and as likely to end--or succeed--as monogamous relationships.