... not--I had to guess that I'd be happier and fortunately for me I was right; it's among the best choices I've ever made). There's even an idea that's at least similar to what you describe: the Peter Principle says that in an organization you rise to the level of your incompetence (e.g. you're good at sales so you get promoted to manager, which you end up sucking at). That certainly sounds like a recipe for unhappiness to me.

In my case, I knew that I wasn't enjoying what I was doing (at all), and that it was unlikely to change since my primary motivation at that job was to make a good living without real regard to whether I enjoyed it. It was even harming my family life since I would wallow in how much I hated it, would dread going in from the moment I left the office, and was irritable even at home. Further, the window to my "dream job" was closing and I've had enough regrets in my life that I didn't want to miss out and forever include it in my list of "what ifs." I knew the job I was looking at would provide me with a comfortable but never ever wealthy life and I decided that would be fine if I enjoyed it more. So I took a (long) shot that paid off and I'm living my dream.

Maybe you need to downshift to your proper skill level like you suggested. Maybe you need an industry change. Maybe the problem is actually all internal (depression-related) and you would be best served focusing on fixing that part of your life. Here's what I do know, from experience: if you genuinely believe there's something better for you out there, do it. It's hard and scary and you'll probably second-guess yourself along the way but I'm telling you, don't sit around wondering and wishing if there's something you really want. And if not, if you can really be content with what you're doing now if you were to fix some other stuff, then do that.

Life is way, waaaaay to short to be miserable, my friend. Find whatever works for you--new job, new industry, new hobby, whatever--and make things better. I promise it helps.