Anon12 - I can really strongly relate to how you feel. There's a sense of injustice about being with an A for ages, supporting them when they need it most, always being there, never getting anything in return and one day - when they feel better - they decide they don't want you anymore - and quite possibly move onto someone else. It's a massive kick in the guts, and shows a lack of loyalty.
Unfortunately I don't have any words of wisdom for you. But - I do know that at the end of the day, even if I don't want to believe or feel it, I'm better off alone than being with someone who treats me like that. I deserve better, and so do you.
They say that success is the best revenge. Not that you should inherently be seeking 'revenge', but being happy, content and not relying on someone else to be your source of self esteem is going to reap dividends. And being happy, positive and assured in your own skin will mean that you attract similar people!