Check out Effortless Mastery, by Kenny Werner
This helped me loads after becoming creatively blocked after four years doing music academically.
Playing music that's totally different from what I would normally do has helped me improve on the instrument more than I would have expected (playing accomp for vocal groups and stuff like that), plus it's fun because there's no emotional attachment to it like there might be if it was "my music". Later on I found that some technical aspects of my playing that had been underdeveloped improved with this type of work, whereas if I had focused on "I need to get better at this" and forced some kind of exercise it might not have integrated as deeply(?). It also ended up being really fun and I feel a lot more comfortable playing different kinds of material than I did when I was overthinking everything. And I guess my point is that by broadening my practice this way, and improving on a technical level, I also loosened up creatively.
As for tedious and mechanical practice sessions, some good advice that was given to me: Always end your practice session (especially the gruelling ones) with something fun that you're really good at. This way your last memory of the session will be more positive, vs walking away in frustration or whatever.