That sucks big time.
My regular partner is similar..
We've been playing together for three years now since i took up the game. He had already been playing for about 15 years at this point and playing off about 15 .. I've gradually brought my handicap down over this time whilst his stayed at 15. He has always taken the odd shot off his score on bad holes but recently my handicap has gone down to 12(through lessons and hard work) and his has miraculously gone down to 8(through deceipt)
The other day i caught him cheating on three separate occasions.
1 count of : Miraculously finding a ball that had gone out of bounds 'In bounds'
1 count of : Improving his lie by moving the ball with his club head at address
1 count of: 'Finding' his ball in the rough and playing the shot then walking forward and finding his actual ball and putting it on his trolley. (The only ball on his trolley and easily identified by the make and his initials)
And that's just the 3 times I saw him do it.
If he's that desparate to win perhaps he should find a new partner who's a beginner.
Also, I've only actuallly beaten him once previously because all the other times i've opened a can of whoopass.. he's picked up on holes to void his card.
He is also reluctant to play in competitions.. presumably because he knows if he gets caught in a comp the person he's playing with will say something, where as I won't because it's not worth falling out over.
In short.. you are not alone