Hi,
Thanks to the poster for bravely describing their situation and asking for advice. I graduated in math from Cambridge a good few years ago, and have a few comments.
These days, I think having a 2-1 from a reasonable university in a good subject is very important. A 2-2 from Cambridge is not perceived as so worthy. This may be unjust, but that's the way of it. Another important goal is to be happy, of course.
So I think you have 3 choices, basically:
(a) bravely plug away
(b) change tripos at Cambridge
(c) change Unis keeping doing math.
(a) Math at University level, particularly Cambridge, is very abstract and different from school. Many smart people find that it's not a transition that they can make. Although, to some extent, study pays off, a lot of it is just "natural" talent. Unless there is some underlying reason for problems in Year 1 (e.g. illness) it's not going to get any easier. Indeed year 3 (so-called "part II") is really hard. The one guy I knew who did try to keep going to the end after an initial 3rd dropped out. So if you've already tried working hard, then it's time to look at the other options.
(b) My friends who changed tripos generally did well. One in particular went from a 3rd in Math to a 2-1 in Psych, and has now got a stellar career in IT. Another got a 1 in some kind of economics/business course - I forget the details. There are lots of other tripos choices available, and there is no loss of face associated with changing subjects - the Cambridge system is deliberately flexible in this way. People do understand that Math, especially at Cambridge, is hard.
(c) Changing Unis. I don't know anyone who did this. If you are enjoying Uni and have good friends, then it seems to me sad to have to leave them all behind. There's also the question of how easy you might find it to settle into a new Uni and make new friends. And you could still be stuck with a difficult Math course. But I can't really advise on this option.
So based on what you say, I would recommend looking at other subjects in Cambridge. I myself arrived at Cambridge to study Economics, but was able to change to Math before the first day of study. It was very smooth, and I'm really glad I made the shift.
All the best.