Be happy for those 18 years of not being single....I'm 35 and I was single constantly....I had five years of relationship which ended up to just one year of marriage and divorce....and I've never felt I had somebody with me...I get to shity relationships and get out of them....this is a life...what can we do for it...
be happy for what you have...your beautiful kids....and honestly, let him go...trust me...after awhile, he will come back to you....that's the way it is....We got to accept the differences between men and women....
Women try to work on the relationship by talking, discusing it out loud and as much as we talk, we don't get anything back....it looks like to push him away....versus, we can just be cool and not talking or discussing and not trying to fix the relationship...just let the relationship to heal by itself....time is the best healer....don't try too hard...it won't work....stay positive and hopeful....and see the result....
Also, try to do excersize as much as you can....it makes you feel better....
good luck...