Sanyi

Sounds to me like you have too much prebend in the mast as stated above.

One measurement you do not give is the prebend.

There is usually a class or builder defined method. But it is usually something like:

Take a line of cotton (or whipping twine) and pull it tight between the base of the mast at the gooseneck or black band if you have one and tape it down. Do the same at the top of the mast by the black band or another "constant" place. Now go to the mast by the spreaders and measure the bend between the back of the mast and the line using a tape. In 15kts, you should both the trapping and starting to de-power with the downhaul.