use your empathic nature to divine the true nature of the actions of those who you feel are manipulating you.
on a more practical note, learn to identify certain behaviors that are indicative of manipulation, and once those red flags are raised, well, the clear solution is to cut yourself off from that sort of negative influence. there is no reason to punish yourself just because you are altruistic and sensitive to the emotions of the world around you. a lot of it also comes down to some amount of confidence, but then the problem also arises that confidence is bred by success. the point is that sometimes, you just have to fake it until you make it. hard advice to follow, and hard words to swallow too...
people will attempt to take advantage of you for as long as you allow them to. it is hard to impress this upon people, but choosing to associate with people and still be pleasant with them even when they have no respect for you or your desires, breeds the idea in their mind that it is okay to take advantage of you (or that you won't do shit which really adds up to the same thing)
no-one deserves to be used. this sorta hits home with me, but for different reasons that i altogether don't feel are necessarily a grand idea to expose to the world at large.
another piece of advice i would have would be to learn what your true value is; learn to disregard your idealized self, and learn to discard the self that these people who take advantage of you have created. identify who you really are, i find it provides extreme solace and comfort to be self-aware of your positive traits, not just your negative ones. don't feel bad about your situation though; it can happen to anyone.
you can't always just feel bad for someone and do what they are asking of you just because your empathic nature allows you to feel their (supposed) pain. it's not your job to solve the problems that everyone else has; i'd find it a more appropriate approach to find some value in putting yourself before others sometimes. i'm not necessarily advocating selfish motivations, but i am saying that there's a certain amount of selfishness required in living.
i feel like i've talked around the point. sorry if i did.