NO!!  Lenten sacrifices/promises are self-imposed which means there is no sin involved in breaking them.  We should certainly try our best to be faithful to what we have committed ourselves to do during Lent, but we needn't be burdened with guilt if we fail.  When we do fail in keeping a lenten promise, the thing to do is to forget the failure and contiue trying our best to be faithful to our committment.