I may be shouted down here, but her goes anyway. Unless you have a very open relationship with your GP, I doubt they could really help, sure they might be able to prescribe drugs or something - or send you to a psychiatrist, but thats not really helping.
All the best advice says see your GP, but personally I dont know my own GP from the next person. I do have some kick-ass friends, who can tell when im getting depressed about stuff. They always show they care, and even when that doesnt immediately cheer me up, its important to know later.
A couple of handy life tips that could apply to anyone, depressed or not:
When you wake up in the morning, have a look at nature around you, the sun the sky, the grass. Take 5 minutes to notice something different about your surroundings. You can then take that thought with you all day.
When your feeling specially down, force yourself to smile or laugh. The very act of doing that starts the good endorphins flowing and make you start to feel better.
Before you go to sleep at night, think of something nice that has happened during the day, someone who said something nice to you, the way something looked, the feeling you got as the sun hit your face - anything will do. A happy thought before sleeping will get your mind lifting itself up without you noticing.
Now, it sounds like you really miss ur ex. There's nothing that can cure a broken heart better than being loved. I dont know what happened between you, or what will happen, but socialise with the boys(even if your not interested in them or if they have girlfriends). Socialising with them doesnt mean you have to love them or jump into bed, but it will get you more accustomed to other men, making it that little bit easier to meet the right one, after all, he's just around the corner!