IMHO...if you have reached that stage where you feel your life is in crisis.....STOP!!!! Look at what you have, who you are with, where you are going.....think about how you can change things without harming yourself or anyone else....it's not just your life..it affects all those around you also.
Two of my male friends have lost their lives after rejoining the biker set in middle age, leaving wives and families in bereavement.
A few men I know have left their wives and a woman friend her husband for someone younger...none of them have ultimately found the happiness they thought they would.
Enjoy what you have...and if you must change things...don't do it on a whim...you are old enough and responsible enough to know better!
If you want to change things, please just aim to change the things you have for the better...actively work at all you have to improve it...that way no-one gets hurt. Share all the new joys with the ones you love..not furtively behind their backs.
From someone who has been hurt.....Best wishes,