Times when you have to "stop moving" might be when you sit down for a coffee, you go to bed, etc. You can't distract yourself 24/7, and fill the rest with meditation. Washing the dishes - you can try and do it "mindfully", but sometimes, it's just you and your head and not much else is going on to distract you.
Stuff that gives your life meaning is anything that feels important to you. Volunteering is great. And I'm studying, which helps a lot. But it's as mich the smaller human things that make you feel like part of humanity. I spend 1-2 hours a week at my grandparents place, and fix their garden while I'm there. I know it's important to them - they really appreciate my visits. So that's become meaningful for me. "Big" meaning is huge, but you don't have to set out to change the world, just do things that are relevant in some way. After a while, they start to give your life meaning. "I matter to my grandparents" isn't curing malaria, bit it still gives me meaning.
It gets easier to not dwell on the thoughts, and let them pass, with practice. It's like an addiction- your head is used to the thoughts, they're familiar, and when you act on them, it reinforces everything for your brain. All the negative beliefs about yourself, including the beliefs like "I have to do this", "this makes it feel better/appropriate for a while", "this is something that I can do that I control", etc etc.
If you can start not acting on those thoughts, your brain gets used to the idea that they're just thoughts and there isn't any sudden expectation of relief or whatever you get from it. The more you don't act, the more they become "just thoughts", and gradually it gets easier for your brain to let them go. If they're just thoughts, and you start to know you're not going to act on them, there's no reason for your brain to stand to attention every time they show up, it's just, "oh yeah, there's those f***ed up thoughts again..."
Every time you self harm, you reinforce your brain's belief that this is something you need to do. You make the process harder. So you start by denying your brain the hit that it's after. Like any addiction, it's near impossible at first, but it does get easier and easier the more you retrain your brain that actually, you don't need to do this. You can cope without it.