Yikes, well this is a bummer to read. I don't think you're growing up tbh. It is normal to have times where you are in a block or start doubting something you are excited about etc.
This used to happen to me and what I would do is take short absences so that when I came back I was ready to design after the break. So like not being on for a week helped. OR not being on every single day. I mean i'm not asking members to stop being on every day but it sometimes can wear you out.
I think you find something you REALLY want to do, and do it. Like for me it was finally realizing I was good at furni and making that furniture store inspired me to keep going. So find something out want to make or maybe practice on getting better at. You don't have to constantly put out content.
Also so many cool features are hopefully coming back and maybe that will inspire you. Just hang in there
I'm sorry to hear this, but I understand how sometimes interests change overtime. I often get into creative slumps where I don't feel like drawing or designing anything. The good news is that usually it just comes and goes in waves and often I find that after a short break I later come back with more enthusiasm. I hope you don't get overly discouraged about the studio, I know they kinda changed a lot of things about that over time. The good thing is there are plenty of other solutions out there that can be used and work just as well if not better. Honestly I'd suggest GIMP since I think it's the closest to Sumo and pretty straight forward. Either way its good sometimes to take a breather and you may find that more ideas will come to you in time!
Never grow up totally Art! Keeping young will be the one thing that keeps things from becoming dull. Mojo comes and goes but it won't come back at all without the innate spark of youthfulness. Trust one that's been there and back! lol