JMO, I am so sorry! Everyone here knows the pain you are experiencing. You are very strong to be able to apply the DB principles so quickly.
I would celebrate your children's graduation as you otherwise would have.
Also, I would talk to a lawyer asap. You need to know your rights in case H goes on with the divorce. It doesn't mean you have to file, but do consult because you need to have this information and it is going to be so empowering.
What did he mean by 'giving you until the end of the month'? Is there something he thinks you're supposed to do to possibly change his mind?
If he is not staying at home and you don't know where he is, I am sorry to say that I think he is likely having an affair.
My H said very similar things to me - had a list of things from the past that he had been terribly hurt by, all reasons to split, and I had really not had any value in our 15 years of marriage except help run our business, etc. We were just so different, there was no way we could make it. Most of us here have heard the same thing. It's nothing you did, they are trying to find a reason for their hurtful actions so they don't have to feel so guilty.
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17