Sex with a 20-something in our 30's?

I am 34, my partner is 29. So my partner has been complaining a lot lately that I'm "somwwhere else" during sex that I'm not getting off and I seem as thought I'm faking it. Not at all the case. I can't get off as easily as I once did. She's been trying so hard to get me in the mood and when we are having sex to even get me off.  I adore her, I'm just not getting there like I did. I feel my body struggling. I know I've completely slowed down. What can I do? Is there anything anyone can tell me to help me?