Big Mama,
I used to have to dissociate regularly with my ex-hub and ex-bf's in order to make it through sex. I was with my ex-hub for so long, that the dissociation and misery became worse and worse as time passed. First, it was fellatio. Then it moved onto penetration as well. After 10 years (or so), I couldn't even handle kissing my ex! I felt horribly guilty, like a terrible wife, for being so negative ~ but that's just the way that it was.
A year or so after we separated, I became involved with my current bf. He's taken things much more slowly ~ and I've been the one in complete control throughout (as my bf is paralyzed chest down). that has slowly allowed me to remain present emotionally throughout the experience as well as enjoy it.
It was a tough road for me...you have my sincere sympathy!