Breaking the boards

Maybe a known question but…
Today we are interested in the breaking of the tablets (among other things), but it is written that they said to Moses, “Blessed be your strength that you broke,” so breaking the tablets is a good thing. So why do we fast about it? And if we fast about the fact that it was necessary to break them because of the calf (in other words, bless your strength now in retrospect, but in the first place it would certainly have been better if we had not had to break them), we should have said that we fast about the calf and not about breaking the tablets?