I'm not sure how long I can put her through much more of this. My wife has been incredible for the past 4 years
Nothing compared to the trauma she'd have to deal with should she ever lose you mate.
Think of it like this, roles reversed would you do the same for her for as long as it took and as many times? I'm sure you would and without hesitation.
It's a bit clichéd but the philosophy of 'this too shall pass' has to become the cornerstone of any mental health battle. No emotional state, good or bad, lasts for ever. It's just that some people's peaks and troughs have greater depth and length, sometimes it feels like it will never pass, but it will and does.
The key is to start recognising the patterns of when that low may be starting, did something trigger it? A time of year, a bit of weight gain, money issues etc., and try to identify patterns.
At the same time, what patterns do you notice when it starts to lift? Some time off work, going away for a few days, a boro win etc.
Mental health is often a life long battle, and it's about trying to be proactive in identifying trends to be able to make the ups and downs more manageable and smooth.
Other practical steps are exercising consistently (can be a 20 minute walk on your own listening to some music or a podcast), deliberately making time for doing things you enjoy every week (selfishly, unapologetically and for no other reason than you enjoy it), get the Dr to test your bloods (low testosterone amongst other things can be a big problem), and obviously medication and talking therapies.