Been guilty of hitting the accelerator on occasion as well. Sometimes the muscle memory takes over.
As Matt Hayden says, use a metronome. If the clicking is intimidating or bothersome, you can get more sophisticated metronomes. Check out the apps available for smartphones and a thing called the Pulse made by Soundbrenner.
Backing tracks are cool. But they are limited to other people’s songs, and only those songs that are popular enough to make them feasible.
Don’t let the singer carry the tempo. They can set tempo by counting off, but one of the cool things about singing is pulling and pushing the beat to emphasize the lyric. If you follow that, you’ll be speeding up or slowing down accordingly, and they need you to keep time so that the rubato has a beat to work against.
Keep it simple. If down strokes on the 2 and the 4 work best, then don’t embellish. Guitars have a lot of sustain. You can use that to your advantage.
Playing with drummers (even if they are not playing a drum kit) will be better than playing with violinists, harmonica players, banjo players, etc. This is because those instruments are really not keeping an obvious beat.
Hope this is helpful!