Is it possible to fix a shower faucet?

It is possible to repair the mixer, in principle. But first you need to understand what exactly is the cause of its malfunction. And what kind of mixer do you have? If the water pressure is weak, then the problem may not be only in the mixer. Or maybe your drains are clogged. Or buy a new faucet and see how it works. In principle, the easiest way would be to call plumbers. There are proven guys who work in Toronto - https://pomplumbing.ca/drain-repair-installation/. Probably, it will be faster, easier and even cheaper.