"This might sound crazy but some men like the brainy type. When I was 20 nobody thought I was smart; I was a hot piece of ass and was reminded of that every time I stepped out the door. Now I am seen differently. I prefer it actually. I still take advantage of every advantage I can take in the looks department, but men want me for my mind and energy. Not an army of them, but a few. But I don't expect to find a rich genius with a six pack. (Except maybe a six pack of beer!)"
If you were really as smart as you think, you would realize that intelligence isn't age-dependent. Wisdom obviously is. But as a percentage of the population, there are X% of the women at age 40 and also X% at age 20 that meet whatever IQ you define as smart. Further since there are more single 20's women than 40's women, it's actually easier to find a 20's woman that has an X% IQ.
So really, what do you have to offer moreso than a 20 year old woman? Maturity? Life experience? Wisdom? Fine I would acquiese to that. Personally I'd rather have bright young hottie with energie to spare and a nice tight body than those above qualities a 40's woman might offer (but also coming with the baggage and saggage), but to each his own.
One more note, even if a man chose to marry you because he found you slightly more desireable as a 40 year old woman than h did another 20 year old, would he feel the same in 10 years when he's married to you and you're 50 vs what could have been his 30 year old wife? 60 vs 40?
Good luck but I think you're in about as good a position as you're going to get if you're already married.