If you are financially stable enough on your own or after the divorce, you are NEVER too old to get divorced & start your life over even with grandchildren though it would be a lot more difficult caring for them all by yourself, but then it sounds like in the circumstances you are in, you have been pretty much caring for them yourself anyway if he's finding time for this other woman.

I am 63. Left my H 9 years ago. There was no infidelity in the marriage on either part but there were behaviors that had been serious problems from the beginning of the marriage that I only tolerated because I was busy getting my degree & working at my computer design engineering career & when I was around him it was fighting but I was away more than around him so it was much easier to tolerate.....something he actually said he thought I would do for the rest of our lives. After my career fell apart when aerospace crashed in California & I couldn't get hired by any other company & I was dealing with burn out at that point in time also I ended up at home & there was no place to escape. I actually lived under the same roof but separated for 13 years before I finally was financially able to leave.

Best thing I ever did in my life & I have finally been able to learn who I really am. I understand it's easier without having grandchildren but not being able to trust your H & then basically just having him as nothing more than a room mate, I know is not good living conditions for anyone.....& it's definitely NOT a good example for your grandchildren.