Mrs. Mallard's repressing of her joy says much more about the culture and society in which she lives than it does about her own character. Although she is certainly thrilled on the inside, she feels the need to play a part and project the kind of sadness and misery Victorian society would expect from recently widowed woman.
Revealed in the irony is the inherent conflict between feminine identity and stereotyped gender roles prevalent in these times. Louise Mallard walks a fine line between these two worlds and represents the tradition against which Kate Chopin rebels.
I think that you answered the question in the wording of the question. The key operative here is "repress." Mrs. Mallard is a social being whose fundamental premise is that of repression. Women of the time period that Mrs. Mallard is a part is one where repression of voice is synonymous with consciousness for women. Louise's struggle indicates a battle between what she is feeling and what she experiences on a subjective level with what is experienced and dictated from a social point of view. This battle between what society expects and what an individual wishes helps to constitute the primary conflict that Louise experiences in the short story, in the hour.
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.