Both the rosary and the scapular are private devotions. Neither one binds under pain of sin.
Please don’t ever make a vow before Almighty God either to pray the rosary or to wear the scapular. I’m not sure it’s even possible to do something like that without the guidance of one’s confessor or spiritual director. It is a wonderful thing to be devoted to the rosary and scapular, but these should make salvation easier, not harder.
Just do the best you can. I personally try to make up “missed Rosaries” and such, but as HS Dad said it is not a sin if you don’t, and sometimes one simply can’t make up all missed prayers and has to just forge ahead. Remember that a prayer and devotional life is a work in progress, a learned art and will have some ups and downs. If you find yourself consistently missing prayer for a certain reason (for example, you fall asleep praying) then try addressing the issue perhaps by saying shorter prayers or moving them to a time of day when you’re more awake.
She’s very understanding. I wear the Scapular too and am in the Blue Army. I’m supposed to say 15 decades a week. Usually I do say the 15. But some weeks I say less and other weeks I might say 20, or I might even say the 5 decade Rosary every day. I figure it evens out. She knows I love her and I try not to slack off.
If it is no sin — and it is not — there is nothing to confess and nothing for which mercy is needed. I have a very real concern that this could lead to scrupulosity, not necessarily in the OP, but in anyone who could come away with the impression that it is a sin to fail to honor a promise made to Our Lady. (That would all depend on intent. If someone said “I’m not keeping that promise because devotion to Our Lady is stupid”, then that would be evil. If someone said “I really don’t think I can keep that promise anymore for good reason XYZ”, I fail to see the sinfulness in that.)
Aside from promises not to sin again, I shy away from making promises to God, or to the saints, under any circumstances. I just live my life, and trust that His Will is done in me. And I never say to Our Lord “if You do this, I’ll do that”. I’m not comfortable with that kind of thinking.