Never force a friendship when it is falling
Some friendships last forever and some don't. True friends are there for the good and bad times. Friendship however is always 50/50.
If one person is putting in all of the work it is not friendship. That also goes if a friendship is going stale. If your friend dumped you for a guy or another girl then that friend is not the friend for you.
You may be upset and wishing that it all will change. Of course you will be hurt but the more you hold onto something the worst it gets.
In a situation like that it is best to let that person go. I know easier said then done but at the end of the day all it will do is get you stressed out. A good friendship is when you are both happy. A good friendship is pretty much easy. Sure all friends fight but true friends stay by each other's side no matter what.
This friend may be important to you but is it worth getting stressed over? The way you met this friend you will make another friend. Don't force anything though. Get to know people, join activities and when you least expect it you will find the perfect friend.