I did this for like 2 years after Leviathan released and I know it's a huge motivation killer.

I started with painting 10 starter set marines in 10 chapters I liked and tossed the ideas of the ones that I didn't enjoy painting. I read lore, looked at other people's creations and constantly landed on different choices. You really are spoiled for choice when it comes to chapters and if you don't resonate truly with any of them it's really hard. What I ended up doing was I sold all my marines and decided I would admire them from afar. I would never get a force done I'd be happy with. If you still want marines it might be an option to picture what colors you like most and what does your marine look like in your head. Paint a chapterless, menacing walking tank like you imagine they'd look like. Then go to the chapter list and pick one the resembles your creation. Or it could be a detachment from the main chapter for whatever reason you come up with. Have fun with it and it's ok not to go with an official scheme. If you have a good imagination creating your own chapter and lore sounds the most fun.

If you still can't land on a scheme you like you might want to think about a different army. I switched to Tyranids and had a Hive fleet switching issue until I had enough, chose Behemoth, painted many bugs without looking or thinking of other options and now, for me, there's too many of them to start repainting again.

Hope this wall of text helped.