Hello Jim
As Victor says, you are not alone in the way that you feel ~ just looking at some of the other posts will show you that.
It is a good idea to talk about this ~ or anything else which bothers you ~ with someone you trust. Is there a school counsellor or a teacher you get on well with, perhaps? Maybe an older brother or parent?
Being a teenager is a wonderful but difficult time. Emotions are heightened for various reasons. This means that the good times seem extra good and the bad times may seem really terrible. Bear in mind that this is temporary. Things will improve. Honestly! Those of us who have left our teens behind can remember them as bitter-sweet days.
There are very few people who meet 'the one' at 16 or 17, but it can happen. I don't know why you and your girlfriend broke up. Maybe an argument or a misunderstanding, perhaps. It might be worth you sending her a note, as Victor suggested. That might sort things out. Or it might just be that she is too young for a long-term relationship. At 16, that would be reasonable, I think.
Try to get involved in other things ~ sport, chess, football, whatever interests you. You may win her back or you may find someone else. Either way, in about ten years time, you will look back on this time with a smile. By then, you may be married to someone else and have a few kids. When your son is in your present position, you will be able to give him all this kind of advice, but he may not believe you, because breaking up with a first love is difficult. Time heals! It really does.
If you are able, try writing a poem about this. That can be very cathartic.
What does the song say? First love is the sweetest? Yes, and it will always stay with you, but it is just one part of life's rich tapestry.