OP, I do feel for you, and as some others have suggested, it does sound like this is not a very happy marriage. I am not going to suggest that you leave your (D)H, because I know that if I was in your position, sorting out all the problems of leaving my husband under these conditions would seem far too exhausting and problematical, than just living with the situation.
However, I would also think, it is "only" another 8 years until my youngest is an adult, and whilst trying to keep as amiable a relationship with my husband during that time as possible, I would also be squirriling away a little money each month if possible, and doing some longterm thinking about where I would live afterwards.
If I am wrong about the state of your marriage, and that you do still love your DH, and believe that he still loves you, then please consider marriage counselling together, because you both need to feel not only loved and wanted, but respected too. I can almost promise you that having an experienced, and not biased other adult in the room, aiding your discussions is very helpful (through personal experience).
The way you describe your (D)M&D OP sounds to me like you have never felt that they put your needs and feelings above theirs. I don't want to sound like a psychologist, but I wonder if you always putting your DC first, is as a result of feeling that your DM&D never did that for you. Anyway, as a (hopefully D)GP myself, most of the time my DH and I have spent looking after our DG has been to suit their DM&D, but twice a year we have taken our DGC on mini holiday's at our bequest.
So, I think that you should ask your DM&D to have the DGC this time, and see their reaction, I am hoping it will be one of delight that they can both have their DGC, and that they can help you out too!
For the present predicament, please do also think seriously about the suggestion some other PPs have offered, about you taking your DC away somewhere nice for the 3 nights. When we take our DGC away for mini breaks we (now that they are older) book 2 connecting Premier Inn rooms, and take them somewhere that is about 2 to 3 hours away. In the summer this could be to near a seaside resort, but for us it would be somewhere like Alton Towers/Chester Zoo/Legoland/London Zoo, plus visit the sites in London, etc If you all enjoy whatever type of activity you choose, and can of course afford it, you should have a great time. When I was their age I would have enjoyed visiting anywhere different, and staying in a caravan (especially if it had a pool), and on rainy days playing board games in the caravan, and getting all wrapped up, and going for a drink and cakes to a nice cafe. If I could afford it, I would have actually prefered that, to staying at work whilst my DM had her DGC.
Please come back OP when you have decided what to do, I and I am sure quite a few others, do care what happens, and hope to find out that you have decided on a course of action that you are all happy with xx