Open Question: Should I wait to be dismissed, or hand in my notice?

I've worked in a store for the past year. I've never been particularly good at retail but desperately needed a job. The trial period lasted 6 months and, obviously, I was kept on after this. On Sunday, I had a review. In this review, my manager informed me that I was the "most unproductive member of the team", "other staff have no respect" for me, my attitude is "too negative" (it's improved massively in the past few months, but apparently he doesn't like that and wants me to be miserable again, hence telling me nobody respects me!) - and he wants rid of me, basically. He also told me that if he'd known that my "babbling" in the interview was just me - and not down to nerves - he would never have employed me. I've got two weeks to improve, otherwise I'll be given a warning, and taken down the disciplinary route. I've tried to improve in the past, and it had no effect, so I'm almost certain that the disciplinary stuff will happen. What should I do? Wait for the disciplinary, and probably lose my job? Will I be given a terrible reference? Will I be entitled to benefits? Or should I hand in my notice, have a reasonable reference, and have to last a month and a bit before I can claim JSA (I think it's a 6 week wait for leaving voluntarily)? He told me he doesn't want me to hand in my notice - that's not what he's encouraging me to do... but what is he doing then? Telling me to wait around and potentially ruin future job chances for a petty weekend job? I don't understand why he kept me on after the initial six month period - I've not got "downhill" or anything - and he must have realised in those first six months that employing me was a mistake (re: the "babbling"!)... I think it may have something to do with limited hours available, and too many staff - I'm probably the easiest one to get rid of. I am currently looking for another job, but obviously I'm struggling. I think I'd be better suited to admin, but with no experience, I've got almost no chance of finding anything. I'm planning on going back to college or university in September, so hopefully things will improve - but I need something in the meantime to help me pay bills and my overdraft. Let me just point out before I stop writing that I am not the type to laze around and do nothing - I do try to do my best. I've been late on one occasion in the past year, and even that was only by a couple of minutes. I've only had one weekend off and two random days (holidays reset in June, so I've got two coming up in April and one in May) - and I've never taken a day off sick. I'm polite, I try to help other staff wherever possible, and customers like me. I'm not as terrible as he makes me out to be! Any [sensible!] advice would be greatly appreciated :) (I'm 20, if that helps. This job is my only "commitment" at the moment - I'm not doing any courses or anything).