I need a polite reason to quit my job.

I’ve been a PCA for a quadriplegic women for 9 months now, I thought everything was going good, but recently felt like she wants me gone. I looked at her text she sent, and was upset when i saw my name pop up calling me a ditz and an air head and stupid.
I want to quit and work for some other clients that appreciate me more. but I feel bad I want a polite excuse just so the next 2 weeks aren’t awkward(considering the jobs i do for her, awkwardness would suck).
I think I’ll end up telling her on the last day I work its because I know she wanted me gone since November. I was thinking of maybe saying my college schedule came back and its going to be really stressful and I think this job is too much? Do you think that’s ok? its partly true…
I don’t want to quit but I have other clients who really appreciate me and want me with them full time.