How would I leave?

It sounds like you’re unhappy to if it’s been something you’ve been considering for a year. Have you spoken to him about it? Can the marriage be saved? Counselling?

any time we have a big argument he always talks about us being done and it's always me that fights to make him realise how immature he's being saying we're over in every argument. I know he won't try therapy as I mentioned it before.
We've had a lot going on with our son this year and I've told my husband he made me feel insane (our son is awaiting diagnosis but noone believed me) but I've never had an apology for it - I was off work sick and quit my job
Just feel like it's me doing all the supporting and I get nothing back.
He now says I don't communicate with him how I'm feeling but I feel I can't mention anything to do with our son getting his diagnosis soon.

Sorry to hear this.
It’s never going to be easy leaving someone but if you truly believe it’s for the best and see no other way then I hope you have family and friends to support you.
I divorced my first husband due to constant lies and gaslighting and I just couldn’t do it anymore. I gave him 5 chances and then just weeks/a couple of months later I’d find out he had lied about something again. It was drugs and he knew 7 years before when he met me my views on drugs. I’d been away at a hobby of mine and came home in the early hours to find he had left traces and that was it that time. I told him to leave and then I phoned his mum and once I’d told her it all I felt a bit stronger with my choice (we got on really well) then I told my mum and over the days the rest of my friends and family. Thankfully we hadn’t managed to conceive so I didn’t have that to consider but honestly when you’re done, you’re done and you will go through waves of feeling guilty but stick with it