I can so relate!!! Same thing over here. How do I channel my frustration...well I just came from the gym not too long ago, one of the best runs in my life!!! The entire time I listened to empowering "I will survive" type of music and just ran, ran, ran.
Also, get busy. No, not like that

. Just kidding!!! But what I mean is, just do other things to occupy your time so you aren't sitting doing nothing, letting your mind wander comforting yourself with food. There's a quote floating around on here, I can't recall who's it is (someone please credit the originator if they read this) stating, "If hunger isn't the problem, food isn't the answer". DO NOT allow yourself to use food as your comfort, it's more than likely what got you on this journey.
Also, I noticed you said the relationship was feeding and driving your weightloss. I'd just like to say, first and foremost you should be doing this for yourself, yes I understand that the relationship and impressing him may have been one source of motivation, but I just wouldn't want you or anyone to forget the MAIN reason why they're doing this. Just how you used the relationship as a little source of motivation, channel the breakup the same way. It works for some, not for all, but try it.
Lastly, i'll say I honestly don't feel bad about ending my entire situation at all because I look at it two ways. One, if I allowed the maltreatment to go on, what does that say about me? That i'm not important and didn't deserve better, which isn't true and I know that, which is why he had to go. Two , you know the old quote, "when on door closes, another one opens", by getting this man out of your life, this person who didn't show you the respect you deserved, it makes room for someone
bigger,
better, and
more deserving of your attention.
Hope that helps!