Need advice on quitting my job?

Hi, I've just started a new job that I've been at for 2 days and I just know it isn't right for me, and I really don't like it.
I want to quit but I don't know to say anything like 'it isn't the right fit for me' because I think it sounds rubbish. Can anyone think of an excuse I can give for why I can't work anymore? (They know I'm going to uni in a few weeks and that I was going to work part time while I was there so can't say it's because of uni.)
I know it would be better to be straight but I just feel like I can't, so please don't just say I should!