Yep. They took and never gave in return. They used you like a tool. When they're finished with it, they just dump you in an emotional "bin". There are such people on this Earth. Even your boss and/or your country's president could fall into this category.

Or...

Your super-willingness to "give" makes people, even the more "ordinary ones", treat you the way you describe it. You only realise you have this feeling once you can no longer give.