Hi Sheba,
I just found this thread and REALLY feel for you!!! Mom is really putting the pressure on you and it makes me think that Wildcat's point (another thread) is a real possibility. Your Mom is so afraid of losing you that she's clinging to and smothering you. She can't understand that she's actually pushing you away. This is where the guilt trips, using your father's health etc. comes in. Wildcat is really insightful.
I'm unsure how old your parents are but Mom really sounds like she relies on you for friendship/security/help. Is your father's health an issue for her? Is she afraid that something will happen to him and wants to make sure she has you if it does? Does she have her own friends to spend time with? Mom could use some therapy, however, you are probably not in a position to suggest this to her. Perhaps identifying what you are responsible for in your relationship with your Mom (respecting her, visiting and spending some quality time with her)and what you are not responsible for (being her best friend, responsible for her or your father's health, taking time away from working on your studies to debate issues that could very well be discussed at a more appropriate time) would be helpful to allowing you to get a better handle on your relationship- what to deal with and what to let go.
I hope your papers are going well, but if not, you may need to distance yourself in order to get them finished. I doubt this issue will complicate that much if you do. Please let us know how you're doing. I feel bad that you have so much to deal with. I wish your Mom would allow you to fly instead of always trying to clip your wings. You're such an intelligent girl and she's missing out on seeing what you're capable of. Hang in there Sheba!