Being imperfect humans, very often Catholics will break the Lenten resolution or promise that they made at the beginning of the season. It may occur in the middle of Lent, or it could even happen on the first day.
Some Catholics may wonder if they sin by breaking their Lenten resolution.
Unfortunately, there isn't a clear-cut answer to that question. It all depends on what it is that you "gave up."
If you gave up something "good," such as chocolate or desserts, breaking that fast isn't inherently sinful. Most of the time you can't sin by having a chocolate bar, unless you indulge to an excessive degree that reaches gluttony.
On the other hand, if you chose to give up something "bad," such as gossip, then you may sin by breaking your Lenten resolution.
The key to all of this is your conscience, which if it is well-formed, will alert you when you sin against God.
If you ever are confused about this area, its always good to consult your spiritual director, or ask the priest during confession. He will be able to guide you and ask you the right questions to determine if breaking your Lenten resolution is a sin, or if it is simply human weakness.
Lent is a beautiful time of the year to work on spiritual disciplines, always striving for Christian holiness. However, we are humans and perfection doesn't come easily.
The good news is that if we fall, all we have to do is let God pick us up again, and recognize his fatherly love for us.