This sounds like me in the past. It does get better.
I started dating a girl, got a job, and moved out all at once in 2012 when I graduated university. I gained a little weight since I started going out more with the girlfriend.
Then I lost my job May 2014, girlfriend broke up with me in July 2014. I started drinking a lot. She and I tried again briefly, I got a new job at a call centre that turned out to be hell on Earth, and we broke up again (boo, past me was really stupid).
I got a new job in February 2015, but kept drinking through summer 2015 until a couple friends expressed some concern about my consumption. I was trying so hard to reclaim old, happy Wallflower that I was going out all the time and trying to... I dunno, reassure myself that I could be fun and cool on my own without my extrovert former girlfriend around too.
January just this year I started really getting back on track with my health. I had hit my highest weight of 208lbs and now I am down 21lbs.
Here is what worked for me. It took a long time.
Do everything in your power to stop thinking about your ex and his fiancé(e). Do not visit either of their profiles. Block them from social media, delete their number, set up a block in your hosts.txt file if you have to. I understand the temptation to want to keep up with them, but it accomplishes nothing besides making you feel bad. Your life revolves around you now, not anyone else.
Remember why you broke up with them. Think of all the awful behaviour you overlooked because you had those rose-coloured glasses on. Surely they had some qualities you didn't like. Obnoxious? Self-obsessed? Flaky? Rude? Prone to lying? Remember how you felt when they treated you poorly. Would you really have wanted that for yourself?
Pick up a new hobby that engages your mind, body, or both. Watching TV/browsing the internet is not engaging. This is a good time to learn a new sport or skill of any kind. I started lifting weights and getting into digital art.
Take advantage of your support network. Friends and family really come in handy during times like these. Let them know if you could use company as a distraction.
Try to keep up with your daily chores and routines. Don't let your house get overwhelmingly messy. Start small if you have to, set a timer for ten minutes and spend the duration cleaning your environment. Get dressed every day, even if you aren't going anywhere.
The difficult thing is that you have to do this stuff before you "feel like" doing it. You'll only feel like doing it if you start doing it.