I strongly suggest you focus on improving your grades in your other subjects - at the moment you have ABCC - so are likely to get ABC/ ABB or similar next year. However, if you spend your time working on further maths rather than your other subjects, you could potentially drop a grade or two - remember that A2s are more work than ASs. ABB will look a LOT better than BBCc for example, and will enable you to apply to much better universities. Bear in mind that universities are rarely that interested in extra ASs - and most conditional offers only ask for 3 A2 grades. Also, a C in AS FM will not look particularly good on your UCAS application/ CV anyway - top universities would be expecting an A/B.
What are you hoping to study at university? If you're planning to go down the maths/ engineering/ physical sciences route, then yes further maths may be useful to give you a bit of a headstart to the course. However, not all schools offer it as a subject and so most universities do not require it for these degrees - and so should teach you all the further maths knowledge required during the first year of your degree.
If you do go ahead with the Further Maths idea, there is not much point attending an A2 FM class unless you are also planning to sit the A2 exams - you will be doing different modules and the ones they will be studying will require the AS knowledge already. I recommend you self teach yourself, then go to a maths workshop a couple of lunchtimes a week (assuming your college offers them, if not pester your maths teacher to help) to ask a further maths teacher for some help on questions you are really stuck on.