What does Mrs. Mallard´s struggle to repress her feelings of joy on hearing about her widowhood tell you about her state of mind at that moment?

This is only an opinion, but to me, this shows us that Mrs. Mallard is in a fairly fragile state of mind.  It is because of this that she is having a hard time maintaining control.  I think it also shows that the joy she is feeling is very strong.

In general, if we are given time to prepare ourselves, we people are quite capable of concealing emotions that we do not think are appropriate in a given situation.  We are less able to control these emotions when they are very strong and/or when our mental defenses are weak.

I think both of these are true of Mrs. Mallard.  She is feeling great joy, but I think her defenses are weakened because of the shock and because she is in some degree of conflict with herself.  Maybe she does not want to have such "improper" emotions.  Or maybe she does not want to get her hopes up and is struggling to keep her joy at a moderate level.

For whatever reason, I think her mental state is a bit fragile and so she is having a hard time controlling her feelings.