Good choice to stay in college

I know where you're coming from with history, I droppe dit after the first year as I couldn't stand it, although I was already doing 4 A levels and only needed to 3 for uni so wasnt a big deal.
To answer your questions:
1. It depends. Are you likely to get the grade you need in it you think? If you really cannot stand it then might be worth dropping it, but you'd pick up a subject and have half the year to do a years worth of work. So if you could grin and bear it then it may make your workload less?
What year are you in actually? As that would affect whether I suggest dropping it or not.
2. It's a personal preference really, I'm quite good at the sciency/maths stuff and so quite enjoyed chemistry but it's all about personal opinion so can't tell you what you'd enjoy. Only suggestion I can make is take a look at if there's any specific requirements for uni (if you haven't already) for the course you'd like and get a rough idea if there's any subjects specifically most unis are looking for that course.
3. Just be upfront about. Say you're not enjoying it and as a result you're finding it ahrd so you're going to speak to your teachers/head of year/whoever about the best course of action and possibly switching subjects. I would imagine they'd be understanding