soul searching... The Art Spirit by Robert Henri... do NoFap. NoFap increases willpower. Exercise on a consistent schedule, gain even more willpower. Use that willpower to learn new things in art, and think more critically about your craft.
Learn what events could potentially stop you from continuing your art. And ignore every other event as they are just trying to pull you away.
We all have negative thoughts in some form of another that come in waves between positive thoughts(unless you're always having negative thoughts, then I'm sorry). Analyze their legitimacy. The thoughts are not us, ourselves, we just have them. It's like in the spanish language, they say "I have hunger", not 'I am hungry.' Are you really losing interest in your art? Or are you down? Are you insecure because you're afraid of losing interest in your art? Is this really what is happening to you?
Look for a cause for these thoughts. There is probably a 'why.' If you know why, you will know better 'how' to deal with them, no?