Can't really answer here but would love to hear some advice as I'm having exactly the same problem.
My background: Starboard F-type 148 in any wind - no problem with rounding up at all
Any other smaller board = problems. Last weekend I rented Goya FSR 125 which is a modern design and has very similar dimensions to Carve 111 and when the wind picked up I was rounding up constantly, very depressing
Tried the other 2 smaller boards in the past in strong winds - the same problem. (one time I couldn't even start - the board would go sideways).
I know from theory that I should put more pressure on the mast foot over my harness lines/boom and I was thinking I'm doing it, but perhaps not enough.
I'm also moving my sail forward and windward a lot but it doesn't seem to solve the problem.
It's confusing, because I'm keeping quite a good balance on a smaller board but still I'm getting so much tired from making it going downwind all the time. How come it doesn't happen to me on a larger/wider board?