I think a big issue with "motivation" is that it will never always be there. Even humans without mental health issues will not be motivated 100% of the time. If we wait until motivation arrives, it may be a long while or perhaps never.

The people who succeed at sticking to hobbies and passions are those who work on them even when there is no motivation, when they would rather be doing anything else. Of course this is harder to do when you struggle with mental health problems. Depression and anxiety in particular can be huge energy zappers. Unfortunately that means people like us need to work even harder to stick with things.

If there is something you've been meaning to do, putting it in your schedule or even writing it on a "to do list" could help to solidify the commitment. You are certainly not alone in your struggle to stick with things.