Trying to make everyone else happy

As I have mentioned before I am taking care of my mother and my parents apartments but what I probably didn't mention was that my dad is always out of town instead of taking care of his own responsibilities and leaving them for me.  I am trying so hard to be the responsible one in my family but it is getting harder and harder to keep up with school, my mom, my own family, and my dad's mess.  I have so much on my plate right now and am having my surgery on the 18th to top it all off.  I feel like I'm trapped in a corner with no where to go, stuck in my parents apartments with no money to move and get away from their stress.  Sometimes I feel like they are trying to keep me here because I am the 4th of 5 children and the only one who does anything to help them.  All their son's have done is steal from them and I am the only one who has been here for them through it all, telling them what choices they have, what they can do to fix their situation, and what they have to do to make it stop.  Now that I have been there for them for this long I feel guilty wanting to leave but I also feel I have no choice.  I'm not sure what to do anymore.  They are very elderly and won't be around much longer and I believe I should just stick with them till the end but I still feel like I am being used and I just need to leave.  They do not support my decision to go to school and my dad just wants me to marry rich and sit on my butt for the rest of my life but I am happy not having a lot, too much money just complicates things, I have been there and done that.  I couldn't be happier with my fiance and my son and I love that our lives are not complicated together, just with my family.  It is really putting a strain on me and my fiance and he hates being around them sometimes because of the way they treat me.  I am so thankful I have somewhere to vent out all these different thoughts because I would blow up on my family and I do not want to do that, they just do not deserve it.