Thinking about it as discipline/motivation is less helpful because you can't artificially just create discipline and motivation, and anyway you DO have motivation to do these things, you've mentioned in your post they make you feel better. So there are actually barriers in your way stopping you from doing them. Having this reframe really helps - if you focus on "discipline" and "I should just do X" then you're trying to use shame as motivation and I find that doesn't work - it just makes it painful to look at which is really counterproductive because you need to look at it to understand what the barriers actually are.
The Ross Greene message to kids "Kids do well when they can" applies to everyone IME. You are doing the best you can and that's all you can do!
Also look at Struggle Care by KC Davis as she has a similar approach.
You could take one item at a time and try to look for the barriers, the best way to do this (e.g. healthy eating) is to look at what you do right now and ask w-questions (what do you eat, when do you choose less-healthy options, where are you, what is happening, who are you with, etc etc) - but NOT "why" - why not why? Because it tends to elicit shame in response, e.g. "Because I'm too lazy" "Because I can't be bothered" "Because I never stick to any goals" etc etc. No, we're not doing that.
If you know what is getting in the way then you can better create solutions that take those barriers into account e.g. if a barrier to eating fruit/vegetables is that you would need to cut it up and that feels too much so you just grab a chocolate bar or some frozen chips instead because you only need to unwrap it, is there pre-cut fruit or veg you could buy? I basically exclusively eat frozen and tinned vegetables, because peeling and chopping is frequently a barrier to me (ADHD rather than bipolar).
If the barrier is that it's too many different tasks, you could try different styles of cooking such as using a slow cooker, recipes that don't need pre-browning, stir fry with everything in one pan, or "roasting tin" type dinners where everything goes in a single tin together, two-pot type hob meals also work for me (a pot of rice or spaghetti + a pot of protein-and-sauce - chilli, curry, bolognese etc, generally using jar sauce to simplify) or ready meals for the days when you really have no brain power and energy at all.
This is not a quick/easy solution so basically you have to prioritise and work on one thing at a time when you have energy to work on it, in the meantime, drop the shame from everything else and give yourself grace and kindness about them. You ARE doing the best you can, you can't shame your illness away.