Depends if you want to do psychotherapy (couch and notepad) or a more clinical role (doing psych evaluations, delivering psychological interventions etc) or something between the two.
You don’t need to become a psychologist in order to deliver therapy. Mental health nurses and trained counsellors can do both of those. Becoming a clinical psychologist is a longer and far more specialised route. In addition, psychotherapy has its own accredited professional doctorate route now which doesn’t require a degree in psychology.
You need to work out which you want to do. Doing a psychology degree is arguably a waste of your master’s funding if you know you want to work in mental health because there are so many careers where you don’t actually need it. And getting started in mental health may be a whole lot faster if you took a route in via nursing or social work. You’re building experience there and getting paid to do it, more to the point.
But all of those things start by making sure this is actually a route you’re interested in and not just an idea that you’ve decided looks interesting. I would recommend getting work as a support worker, care assistant or similar on a part time basis at least. You can start building basic skills, test out whether you’ve got the patience to work with some of the more challenging clinical populations/client groups you could expect to work with in a career in mental health and start to get an idea of whether you understand what you’re getting into. There are a lot of transferable skills that you can gain from these roles even if you never end up working with these populations.
As to whether it’ll be hard to get these jobs, no it won’t be because you don’t need a degree to get these jobs. Volunteering likewise doesn’t require any qualifications.
I suggest you really explore these careers in depth first before committing to any course of postgraduate study. Psychology conversion degrees are such a generic qualification in the sense that they are often expensive and don’t really qualify you to do that much besides the rare and competitive job of assistant psychologist. You can become an IAPT practitioner even without a psychology degree. If you don’t specifically want to be a psychologist, it really may not be worth your time to do one. You only get one round of funding to do a masters so it’s something you need to choose carefully.