I found that I'd reached the same stage. If it wasn't multiplayer with the chance for competition, I couldn't get into it. If you're looking to branch out a bit though, I found just playing through co-op games with the same people I'd normally, 'train' with so to speak, was a great way to get back into the vibe of single player and non-competitive multiplayer.

Then again if you're happy with things that way, then no need to change things up.