I am losing the fight yall

Grrr...why me? Why am I so nice and always setting myself up for something I do not want? Why? WHY???? Okay, when I took this job...my status was On-Call/Part-time. From day one, I worked 6-7 hours every day (most days not taking a lunch or anything)....not exactly On-Call..and sorta not part-time. Okay, then the girl I was told I would job share with, comes back to work when school starts (because her children have gone back to school), and I finally...FINALLY get my Tuesdays and Thursdays off (which I want and enjoy immensely!!!!!!!!!!), so, evidently, it was VERRRRRRY busy on Tuesday, and this girl freaked out. She does not want to do that job on Tuesdays and Thursdays because it is too hard and stressful. She does not want more responsibility which she was given when she came back (because we are doing the same job). So, today my boss tells me that she is hoping I can work those days until we figure out what to do. She said I didn't have to, but I will...because I am nice like that. There was some hinting about me going part-time/full-time, which I really do not want to do. So, here I am, part-time, going back to working everyday....again. How do I get myself into this? Why am I so nice? Why am I so competent? How can someone keep a job when they don't want to do it? Why is that?