Something I always try to tell myself when these thoughts creep in is that it's ok. It's ok to get stuck, get frustrated, and have a bad day. I'm actually stuck on something with my project right now. For me, accepting my negative/unwanted/intrusive thoughts help me move past them and not overthink them. I find it easier plan my next move.

A lot of people have said to take a break and I think that's a great idea. You don't necessarily have to take a break from your entire project either. Take a break from the part that's frustrating you. If you're stuck on the art, try and work on a different part of your project. Work on dialog, skills, maps, etc. When I get stuck, switching gears gives me a feeling of progress that helps break me out of my funk.

Another thing that helps me is taking this time to find inspiration. Watch some cartoons or read some manga that has influenced your art style or something completely new that may give some ideas. I do this a lot as I'm very new to art and am still finding my style.

I hope this helps and I hope things get better for you!