It's ok to be confused, and dealing with this kind of ambiguity is a normal part of life. I'll provide some ideas, but the real moves are always up to you.
First, it sounds like you're not being challenged anymore or able to really apply your skills to the fullest, thus the depression and looking outward for change. You have the right idea to look for change.
More specifically, look at as many jobs postings as you can for a few weeks and get a feel for what is in demand. Select a few that sound like fun. Then work backwards and make a plan to build the skills described in the postings. You can do that with personal projects, open source contributions, or side work.
As far as suggestions, I see two fairly straightforward options. First, as you mentioned, you could try a new language. Personally I'd suggest Rust as it's more interesting, closer to C++, and used in a broader array of domains. But golang is quite common and has some fun aspects. You'll get a feel for the domains as you scour all those job postings (try reading all the whoishiring posts).
Second, you might also consider looking for a more senior role with C++ where you are building something new. Maybe the maintenance and bug fixing is the problem, and you just need to work on the early part of a project where you have greater responsibility for creating and design. A startup will often take a chance on someone with less experience, so don't underestimate your qualifications.
Again, what you're experiencing is normal, so try not to let it get you down. Keep scouting for new and interesting opportunities. The market is kind of slow right now, so it's a good time for working on those those side projects to build experience while you keep searching and applying. You'll be ok.
