Are you kidding me? You really have no idea.
Firstly, doing Physics might or might not help with Further Maths simply due to your module choices. For AS Further Maths you have FP1 (pure module) and then 2 applied modules (of your choice - or probably your schools' choice). In the applied module you could do Mechanics, which if you do Physics, will be made really easy. Otherwise, it will have a minimal effect except for simply having done more maths (which is always advantageous).
Secondly, you stated that 'Further Maths isn't even what you would call 'normal' maths, and is actually really difficult'. The only way it couldn't be what you call 'normal' maths is that you are obviously unaware of complex numbers, thus this new imaginary number line would seem strange, yet, and I know this is being pedantic, you are saying Further Maths is 'abnormal' maths. And out of the 3 modules that you do for Further Maths AS are applied so these could have easily been done in normal A Level Maths, and surely are done by other students so it is the same difficulty as A Level Maths in that scenario.
Also, please do not deter people from doing it because it is 'really difficult' since, and I am sure any sixth formers who are doing/have done AS Further Maths will agree, the pure module- FP1 which is the only thing that makes AS Further Maths different to A Level Maths, is on par, in terms of difficult that is, with AS Pure modules so C2. If anything C3 and C4 are much harder than FP1. That is why a lot of people actually do AS Further Maths and AS Maths in year 12, and then A2 Maths and A2 Further Maths in year 13.
So, to anyone who is currently considering A Level Further Maths or at least AS Further Maths please do not be deterred by rumours that it is 'extremely difficult' or whatnot; yes it is hard but so is any A Level. If you are good at maths and enjoy it, then defnitely give it a go!