Keep busy, busy, busy like @Jeruba said. And by busy we don’t mean drinking and sitting in your house all alone.
Get out. Go out into the sunlight (darkness makes it worse) and fresh air. Force yourself to get up and go for a walk or go to the gym or do some type of exercise at least once a day.
Make yourslef a list of things that you will do instead of thinking about her or instead of dwelling on the “why did this happen.” Sometimes you’ll never know why this happened. It could be a million reasons. Don’t weight down your carry-baggage wondering about the whys.
Make sure that your pantry and fridge are filled with nutritious foods like fruits and veggies and low fat milk and decaf tea and whole grain bread and nuts. If you have a lot of junk food (like chips and candy and cookie and soda), get rid of it now. You will be tempted to eat it (a lot of it) and it will make you un-healthy and you’ll feel worse.
Make yourself a list of fun/interesting things that you’d like to do in the next several months with some good friends (only the supportive ones, not the ones going through the same thing) examples of fun things:
Go for a short hike in your nearest mountains or park
Walk on the beach and throw some bread crumbs to the ducks
Get out your camera and take some cool landscape photos of some funny pictures of your friends
Go check out an exhibit at one of the museums (planes, trains, art, science, space etc.)
Less fun, but very helpful, drag out some old clothes, books and other junk from your closet and donate them to the local charity thrift store
Look up some recipes online for something (healthy) good that you’ve wanted to try for a long time. If you don’t cook, have another friend come over and help you learn to make it.
Go to the Zoo (bring your camera)
Take a short day trip on a train (because it’s fun) to a neighboring city and have lunch and take in the sights
Plan a camping trip for this coming summer (with good friends)
Take in a sports game at your local high school
Go ice skating
Take an art class
Rent some old comedy classics like Young Frankenstein, Monty Python and the Holy Grail and Blazing Saddles (with a group of friends bearing pizza) go super-easy on the beer.
Have a barbecue with a bunch of good friends. Go super-easy on the beer.
Hook up with some friends that have a dog and go walking with them, and tummy rubbing afterwards. Watch out for dog breath, but have a good time.