Always remember that you can't give all you have of yourself to someone else. You can love someone very much, but you have to keep some of you FOR YOU. You have to be your own person, you can't live entirely for someone else. If you give all you have away, what is there left for them to care for? There are people in this world that are completely self-involved and will take as much as you are willing to give them, without giving you anything of themselves. These are very one-sided relationships, and inevitably doomed to failure. The person who has done all the giving suddenly finds that they are emotionally bankrupt, and I think this is what you have encountered. Obviously, time has given you some perspective on the situation, and the next time you fall head-over-heels for someone, just remember to keep an eye on the balance of the relationship. If you aren't getting the same kind of response back, pick yourself up by your bootstraps and move on.
Best of luck!