You just never know about that. You’re right, it’s more difficult to meet someone if you are over 40 and have kids, but it happens.
I know a woman who had a severely retarded child that needed round the clock care. She met someone and they ended up married. The man was very financially secure and was able to provide excellent care for the poor child. So you never know.
For better or worse is the best way but sometimes staying in a marriage just isn’t an option if the welfare of the family is at stake.
Jesus's viewpoint, from Matthew 19:9
"I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery.”What do you define as "welfare of the family"? I can understand her leaving if he was being violently abusive, or otherwise putting family lives at risk.
Yes, you can leave because you are "unhappy". Just don't do it and then still claim to be a Jesus-following Christian.
Now for the matters you wrote about: “It is good for a man not to have sexual relations with a woman.” 2 But since sexual immorality is occurring, each man should have sexual relations with his own wife, and each woman with her own husband. 3 The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. 4 The wife does not have authority over her own body but yields it to her husband. In the same way, the husband does not have authority over his own body but yields it to his wife. 5 Do not deprive each other except perhaps by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. 6 I say this as a concession, not as a command. 7 I wish that all of you were as I am. But each of you has your own gift from God; one has this gift, another has that.Perhaps, rather than being unhappy over the husband's lack of initiative, the wife could try to lovingly seduce him. Perhaps (and yes I know I will get some outrage for saying this) she could watch some porn and get some ideas about men's fantasies about how a woman could behave towards him.