Either the sex is satisfying and interesting, or it isn't. If it isn't, then you're not sexually compatible with her and you should just move on.
Feeling like you should end a relationship because you want to sleep with other women before you die is really kind of stupid. Either the relationship is a good one, or it isn't. If it's a good one, and you want committal, you have to give up the freedom of sticking your penis in any woman that is willing. If you don't want that you need to break up with her and stop wasting her time so that she can find someone who IS interested in staying with her.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."