I feel that way all the time. It's actually a trigger for me... because as much as I now feel old and undesirable and vulnerable, knowing my WH chose a woman half my age and four years older than our daughter, I know that I would never betray my vows just to feel sexually validated as a woman. I'm not broken enough to believe that having another person want to have sex with me makes me valuable. My WH did exactly that. I hate that the only person I turn to for sexual validation is the person who de-valued me. I never feel pretty any more. Ever. My husband could tell me a thousand times that I'm beautiful, and it is meaningless words to me now. OW was, in his words, prettier than me and 'very, VERY beautiful' (except that she actually was quite average)... and those words can never be unspoken.
An awful lot of people apparently will have sex with any one who pays them enough attention - the more broken you are, the lower your standards. My WH's 'very very beautiful' mistress was a half blind, chronically ill, drug dealer from the wrong side of town. Personally, I don't want to wake up one day shaking the ugly side of the 'they affair down' stick.
Do I want to be loved, desired, craved, treasured? Of course. Do I want to prove to myself, my WH and the world that I could have done 'better' than the broken down wayward that I chose? Yes. But life is so short. If everyone leaves, and I will still be left with myself. I can respect myself. I can look in the mirror. I do know who I am and what I am worth.