What if I can't say no?

The reason why I am asking you personally is because I only want an answer from someone who regularly gives good advice, they will be able to understand my situation better. This isn’t an extremely dire situation however. I am simply overwhelmed by how I am constantly being asked for help with situations affecting my friends and family, I don’t know how to say no. Two of my good friends suffer from depression and are working through it, however they need me a lot and depend on me to always be there for them, which is difficult enough. I love helping people, but I just don’t know how to say “I’m sorry but I need to have a little time to myself,” without feeling like a jerk. Most of my friends know me to be the person to go to for advice, I feel like I’m the only one they trust to help them without judging. And for that reason I never stop talking on the phone or IMing advice to people, or going places to help out. I have very little time for myself but people depend on me, I don’t know what I can do. How do I say I need time to do my homework or to just relax when I have a friend who, for example, is in tears because her boyfriend dumped her? Or say this to a friend who is in a foster home and feels all alone? HELP!