I didn't read all the replies.
I've been playing the same guy for 30 years. Sometimes I play his brother.
When I play my friend, we go about 50/50. Truth be told I will more than I lose but we play neck and neck.
Every once in a while someone will challenge the table. My buddy always makes me play the challenger because I rarely lose the table. He, on the other hand, plays his regular game and we'd lose the table.
What I'm saying is when I play the same guy over and over, I tend to play to his level. Step in some fresh meat and I play beyond that original level by leaps.
It's possible that playing the same guy has decreased your game. On top of that, you might expect to do crappy against him so you DO play crappy against him.
Get some fresh meat on the table. Pick a new game. Do anything to change it up so you don't get stale.
And check your cue tip. Sometimes a subtle chip on the tip can throw off your entire game.
On that note, when I'm shooting, I always check the rotation of the butt to make sure I hit with the same clock position all the time. Very subtle flex shifts in the shaft can alter trajectory enough to affect long shots. If you end up with any kind of warp at all, have the warp angle straight up and down.