First of all, you are not being childish - you are suddenly beset with a whole raft of new challenges that you need to try and adapt to. No wonder you feel a bit overwhelmed!
I hope that it has helped to put it all down on "paper" like this - sometimes that can be a help in itself.
Self-pity visits us all, and, if we choose, it can be the impetus to grab life by the balls and look at ways forward.
Firstly, maybe you and your DH should take yourself off to the CAB and check out what financial help you might be entitled to when both your jobs come to an end. This will be a factor in deciding the next step.
Maybe being forced out of a rut might be a chance to look at some new ideas in life - new ways of using this period of good health before you become decrepit to do some of the things you enjoy before it is too late. And moving on to do these things might also help you not to dwell on the DIL and her Mum scenario. And it might make them look at you both in a new light.
Get some medical help with your menopausal symptoms, as you do not want these to hold you back - but sometimes these can seem worse when you have other challenges besetting you.
We spend our lives working and bringing up children and never stop to think what else life might have to offer - this may be your opportunity to do that.
We have taken several apparently unwise decisions that left us financially worse off, but our view was that life was to be lived and we had to grab chances whilst we could - which turned out to have been the right approach as OH has had Parkinsons for the last 7 years.
My strongest advice to you on the DIL and her Mum front is not to get sucked into the jealousy trap (which is what it is) because that way lies madness! My DGC have more contact with the "other" families in the main, because we are less fit, and I have resigned myself to that with a good grace. Different grandparents have different things to give, and the range and variety of experiences that come the way of my DGC are partly due to all this wider family contact. Six months is no time at all, and, especially if your life circumstances change, you will find your niche in your DGC's life as time goes by. Jealousy is an evil weed and will strangle you - you can choose not to let it!
Hold on to the good things; and make some good things of your own happen, now that you will be gradually relieved of the work, work work treadmill.
This is a whole new phase of life and change can be hard - but it can also bring new opportunities.