Nothing you can do except try to stay busy.
You probably won't have an appetite which makes going to the gym hard.
Try to learn some new recipes. That'll give you a feeling of accomplishment and possibly jog your appetite.
Spend as much time with your friends as possible. Try not to drag down every conversation by talking about "her".
Have stuff to look forward to. Even if that means making a small investment to watch or planting something and watching it grow every day. Having something to look forward to is key.
Finally, remember that you will eventually get over it, even though that seems impossible right now.
Hang in there, you'll be aight.