Firstly, I'd recommend starting
here.
Secondly, answering your questions:
Does being an average student matter?: yes. You need to work
even harder. I was an average student - but it mattered enough that I pushed myself - very hard, this is true of a great many people working in aerospace; there may be a few naturals, but very few.
Which is the good one? All aerospace jobs have the potential to be fascinating and rewarding. The question is - which is the right one for you? Nobody else can answer that, you need to do a lot of research.
Can you take a pilots licence? Yes, at any time - many aeronautical engineers are pilots, quite a few professional pilots have backgrounds in engineering. A few of us manage to combine the two.
Is maths difficult? In some areas, such as research, design, testing - yes it's nasty. Doesn't mean that you can't learn it. In other areas, such as production, maintenance, engineering management - it's still there, but rather less painful.
In a nutshell, there are no easy jobs in aerospace - for anybody, regardless of whether they were poor, average or brilliant students. There are however very few boring or unrewarding jobs in aerospace either. If you want to do it enough, you'll make it.
And best of luck,
G