I have three resits, English and History coursework, A2 and a weekend job which is only Saturdays. I have to admit if it wasn't for my one day off a week I would not be able to do anywhere near the work I need to and from the weeks I've worked both days of the weekend I have hated the fact I haven't been able to get a lie in for weeks. You need a day off to catch up with work, and that's coming from someone in more or less the same position.
In all reality having a job and money for uni is pointless if the job means you neglect your studies and don't end up getting the grades you need. Have a chat with your boss and see what they say, otherwise I would personally leave the job, 17 hours a weekend is too much. Even if you could get it down to 12 hours over the weekend, that makes a lot of difference energy and motivation wise.