I'm very sorry to hear of your pain. I can understand your situation on a personal level. It sucks, and makes your whole body and stomach ache, I know!!!
Ok---here's what I would do if I were in your shoes right now.
I would perform a truth spell on the relationship situation between ONLY HIM and YOU (not the other gal). (For truth spells, search this site by clicking on the Spells button at the top of the page...many of them are really good on here). What needs to happen at this initial stage is that in order to have a positive outcome, you need more information on whether or not he is truly the best partner for YOU. So this will clear that up, once and for all. That will be useful knowledge, because if he is not the best person to share your life's journey with at this point in time, no amount of spell casting will work on keeping you together. So it would be helpful to YOU to have that "edge" of knowing.
Second. After you have performed this truth spell, and the truth of the relationship between you and your partner is revealed, you need to decide what your next step is. Let's say it is revealed that he IS the right person for you at this point in your Journeys. You'll want to look into couples counseling, and follow the advice the other poster offered about cleansing negative emotions and talking honestly with one another. Never forget the power of prayer and meditation. Enlist friends and acquaintances for good thoughts, positive vibes, and/or prayer (with respect to individual's personal belief systems). There is Power in Numbers!
On the other hand--let's consider the other scenario. What if the truth spell reveals that this man is NOT the best partner for you at this point in time? You have decisions to make here, too. These will be harder emotionally, of course. If this is the case, I would then perform a clarity spell. This is like a truth spell, but toward your own happy future. Focus this spell on isolating the energy of the man you WILL actually best with, although you may not know of him yet. See what you find out. Sometimes it's a LOT easier to let go when you see the light at the end of the tunnel---that, hey, I have to let this one go, but look how worth it---I will be happy with Mr. X in time.
GOOD LUCK! You are worth true and sincere happiness. If this is the man for you, it will BE SO. If not, you will still find happiness if you SEEK it. I have confidence in you, girl!