I'm unsure what to do now, I feel like I'm at a key crossroads in my life (being over 40) and want to be happy - but I don't want to hurt anyone (my wife or kids). Advice appreciated
It is often not possible to avoid hurting other people in life, so accept that you have to hurt other people sometimes if you want to live your life. Being a martyr is not a successful strategy.
You want to be happy, but before that can happen you have to work out, realistically, what will make you happy:
- staying with your wife for the next 40-odd years (i.e. "for life") ?
- divorce and becoming single again, seeing your kids part-time?
Those are the only two realistic choices.
You don't have the choice to get married to the other woman because she has a say in that - has the other woman said she wants to marry/live with you?
You don't have the long-term choice to stay married but carry on messing about with other women, because your wife will eventually find out and you will get divorced anyway, in a more damaging and acrimonious manner.
You don't have the choice to "find someone else" because that is down to chance, not choice. It might happen, it might not. If you want someone else, you have to get divorced BEFORE you start looking.
So make your choice - divorce or stay and work on your marriage.