I am so sorry, your pain and guilt come right through.
You did not have a choice. When you are a single parent, you must put survival first. If not, their lives would have fallen apart and yours too. Remember that.
Here is the thing. Even if she is self medicating with alcohol, she is a responsible 21 year old girl who knows better than to drink and drive. While you are looking deep for a reason for this, I want to ask you a question. Is it possible she was just out drinking and having fun and got caught? I am not saying she does not have emotional issues, but they don't always lead to this.
I remember being that age. Thursday nights was the big party night when I was in college. There were many nights I could have gotten a DUI and did not (thank you Lord). Looking back it makes me sick to even think about. My parents could have tried to put a reasoning on all of the drinking that was going on, but in reality, I was in college, partying, and having a good time. That does not make it right, but it was not "because" of anything more than that.
I hope she does talk to the person you told her to. I also hope she gets some much needed counseling as it sounds like it would really help her, and you too.
Tight Hugs. We are here with you.