I'm glad that you have discussed the heartache alongside the joys, as this is the negativity that destroys the family unit without people knowing why, when they love their family members so much. I can see that you would hope your daughter could behave herself more because she knows you have had a tough time, but you are looking at this through the eyes of a wise adult and your daughter is seeing things through the eyes of a young person who needs to learn through her mistakes, and in her own rights, take the path of a youngster despite her brothers extra needs. I wonder if you have always had to ask more of her, in terms of discipline, so that the family can continue to function well - that would make perfect sense, but would still put extra pressure on your daughter along the way. In some ways, i feel that your husband is right - your daughter has reached an age where she can make her own decisions and I also support your belief that she needs to realise the consequences - that you are hurt. But that's the lesson that she needs to learn - you are hurt by her mistakes. Having said that, she still needs room to make mistakes within the support of her parents. Perhaps you could send her a note or sit her down and explain something along the lines that you love her and support her, but that she has hurt you (and explain exactly why - not the behaviour, but the selfishness of it) and you feel that she needs to know that, but that you realise she is growing up and you will always be there for her? She may not see the error of her ways now, in the fullness of being 21, but she will store the lesson for later on. And if she repeats the mistake, then the lesson becomes stronger, for later on. It is also possible that if she was very good when she was younger, that she is learning stuff now that is a little delayed, because she was good enough not to test you earlier. I think it would be very sad
if you spend time apart from your daughter over this. You don't need to use the word 'forgive' to your daughter if you don't want to, but if you can forgive her in your heart, then it won't build up as resentment. I wonder if you have other 'forgiving's inside that need to be resolved, which this situation is triggering? I wish you the very best with this, I hope this gives you some food for thought.
I appreciate that you have accepted one answer and that I stepped in. I felt it was important to offer a different perspective. If you feel my answer has been valuable, then please feel free to accept it.