It may also be a factor of how you have trained your dick through masturbation. Do you have "death grip syndrome" and clamp down hard or require huge amounts of friction? If so, then sliding smoothly into a slippery hole will never deliver what your dick has become used to. This is related to what
@Brodie888 said above. You may need to take a break and re-train your dick.
How do you do that? Stop masturbating. Stop doing whatever it is you are currently doing. Only get your pleasure from the sex with your partner that is giving you the problem today. If you deprive your dick of what it's gotten used to, I promise that your dick will start to respond to whatever attention its receiving. After a few weeks or a few months, you will carve a new groove in your brain (and your cock) and sex may indeed become more fulfilling than it was before. Good luck!