Ohhh boy, I know just how you feel... as these ladies will tell you from my "woe is me" posts a month ago, I had a breakup on similar terms with my boyfriend of about 1 year. I was devastated-- the one who I spent all my time with, slept in his bed every night, and who cheered me up whenever I was blue-- was gone from me.
But you know what? I finally got on a diet the very day it ended.
For me, relationships are all about "we." I've been in and out of relationships for the past 4 years, never taking much time to remain single, and I'm finally seeing how happy it can make me to say "I," not "we." I know that it hurts at first when all you can think about is what you're losing, but I promise that with time the hurt will pass and you'll begin to realize all that you are
gaining (and I don't mean weight-wise!

). This is especially true if your relationship was not very healthy/not functioning well (which is usually the case near the end when a breakup is imminent)... you'll be surprised that, once the initial pain passes, you'll feel very free and relieved.
Dieting was my way to forget about my breakup and focus intently on one thing and one thing only: me. Everyone tells you to "do it for yourself" and I know that can be hard to think about when you're still fixating on the "we" that you no longer have, but just have faith that
things will get easier. Within a month, I am able to say that while I am not entirely "over" my ex, I am over the fact that the relationship ended, and I'm even happy about it. I know that you'll be able to do the same, and in the meantime just try to stick it out and keep telling yourself "it's going to get easier."

Send me a PM if you ever need to talk more about the breakup... I know that when I had mine, I wanted all the support I could get until I felt back on my feet.