Most Helpful Guy
You will go through a normal grief period lasting days to weeks, just as would anyone who experiences loss of someone they cared for and who shared a bit of their life.
You'd be abnormal if you didn't grieve. But you are not on this earth to suffer. You can make things better right now by taking charge of your life by continuing your normal daily routine and forcing yourself to maintain a good diet and exercise program. Get plenty of sleep. No alcohol. Now is the time to catch up on your reading, take an academic course, and see those movies you may have missed. Brush up your social skills with the art of light conversation, body language, and appropriate eye contact and smiling so you can start accepting dates with those guys who've been making sidelong glances at you (haven't you noticed them?). Get your hair done. Always wear a bit of red. Get back into the dating game with others ASAP and enjoy yourself. Learn to meditate. Do those things that make you relax (again, no alcohol). Keep moving and keep looking ahead and be happy with yourself.
You will eventually meet someone you like better than old whatshisname. It's a matter of statistics, self-exposure, and taking chances with new people. You have learned from this philanderer and have become a wiser person, so be glad for that.