I did Edexcel A level maths, which you're considering, so I'll tell you how I found it. For context, I got an A* at GCSE in both maths and further maths.
AS level maths was fine. C1 was only a bit harder than further maths GCSE, and C2 was alright. I liked S1 - lots of people complain about statistics but I enjoyed it. However, I had at that point always found maths very easy and whilst I did plenty of revision and past papers, coming out with an A wasn't a surprise.
A2 level maths was a different story, C3 was hard but manageable and M1 was easy after AS level physics, but C4 I found really hard. I couldn't understand it despite the help of friends and my teacher. I did come out with an overall A in my maths A level but I got a D in C4 - the As in other modules saved me.
I think self-teaching maths would be very hard, particularly from a GCSE grade C. At my school, to study maths you need an A in GCSE maths and to have been eligible for the GCSE further maths qualification. There were still quite a few Us in modules on AS results day. Maths is hard, and if you don't understand it or find it hard at GCSE, you'll struggle so much more with the A level.
However, there's no harm in getting a second hand textbook off Amazon or something very cheaply and having a look and seeing if you think learning that would be manageable. There are tutorials on Youtube and if your friends study maths they'll probably be happy to help. Getting an A level tutor also helps some people a lot.
By all means try, but be prepared for a lot of very difficult work that will be very time consuming (and may consequently affect your grades in other subjects).