As you might be able to tell from my avatar, I’m a fan of Commander Keen. If you’re unfamiliar with the name, this was a series of side-scrolling platform video games developed primarily by id Software for MS-DOS in the early 90’s. They’re the first games I remember playing as a youngin’ (hence the nostalgia/love) and imo they still hold up today as solid, fun games. You can grab them on Steam, GoG etc. for pretty cheap too.
I found them challenging but never to the point of frustration. Plus, if I ever got bored of dying or wanted to skip a tricky part, you can always use the GOD mode cheat (press G+O+D) and just fly everywhere on the pogo stick.