I'd agree with Frank about keyboards if you've got some. Ever since I picked up a guitar 30-odd years ago what I've really wanted to be is a piano player like Freddie Mercury!
A few years back I finally got some keyboards. I find it a real struggle playing them, but every time I do put any effort into it I discover it's done amazing stuff to my guitar playing.
Playing bass does the same thing to me. I'm a lot better at bass than piano, but I have to put a lot of work into creating bass parts - listening to the bass in songs that I like the effect they achieve, trying to get in the mind of the bassist figuring out how he/she fits the song. When I'm done with the bass part I was working on, and I pick up a guitar again, suddenly I can make the thing stand on its head.
But I'd also recommend the "taking a break and doing something else" option - put all the instruments down and catch up on the rest of the world. I really couldn't manage this years ago, I'd just obsess on music, getting frustrated, thinking I was cr@p (I wasn't, but I still thought I was).
If you feel like you're a slave to your guitar and music, that you "have" to do it otherwise you've wasted the day ... if it's driving you instead of the other way round... and then it accuses you of being repetitive and ineffective... then maybe try taking a break. Only needs to be a day or so for me. For example, I come home after work intending to do this or that on the guitar because I "must" and I've only an hour til the missus gets in, and then I look at the thing and think "sod it, you can wait..." and watch Eggheads or something instead. Next thing I know, a day or two later, I've been playing like a demon, written a song, maybe even recorded it, and I can't even remember feeling frustrated.
It comes and goes, I just accept it nowadays... The way I look at it, it's like making sure the guitars know who's the boss - me. They're like a bunch of children demanding my attention, which isn't always the best thing for me. But unlike real children, they're just lumps of wood and metal you can ignore without hurting their feelings!
It's different for different people though, the main thing is to find the way that works for you (and it helps having someone you trust, when you feel stuck in a rut, who says "well, it doesn't sound like it, I was enjoying that..." :lol:)
