It could be. As for content, once you understand the basics, it can be fairly easy to apply them in the harder situations. C1 and C2, are pretty good units that you can get your head around, especially if you're self teaching the topics.
And as for S1/D1, most people I know prefer D1, as it's quite straightforward once you learn all the theorems and stuff.(I've only been through half of each, but they've both been okay so far.)
I've learned all of C1 and C2, as well as parts of M1, S1, and FP1, since September in Further Maths, so that's been about what? About 4 months. (-holidays). But I know people in normal Maths classes have only done roughly C1 and M1/S1. So it's gonna take a lot of work, and depends on how much you go through each week.
How good/ confident are you at Maths?