I used to have this same problem, i used to think saying no would make people not like me or see me as unreliable, but with lots of encouragement to test the waters from my T i soon realised the world still goes on whether i say yes or no. people who really need help find it elsewhere and understand, those that are just using you for convenience either get on without help or find another 'mug'.
ok it took a long time to feel ok about saying no and still today i don't like doing it to some people, but that is because i know they are true friends not one sided ones.
saying no is a great way of finding out who your true friends are... i had loads of 'friends' when i was well and did things for them but as soon as i became too ill to do anything they all disappeared back into the woodwork never to be heard from again!
one sided friends like that i now can do without as my true friends accept i help when i can and when i can't they help me.