Thanks for your thoughtful response, Shazerac. Being sensitive about how I approach the subject, I have tried talking to her about it. What bothers me is she feels guilty about the fact she's not meeting my libidinous needs. I don't want her to feel guilty about it...it just is & I love her regardless. She goes to the doctor regularly & matters of thyroid or other factors haven't presented themselves - thank goodness.
Both she & I have put on a few pounds in the last 14 years, so I think body shame might be contributing...or the fact
I'm getting old, fat & ugly!

This might be TMI, but there have been two occasions on which I asked if she could perform oral sex on me. She declined, however; & if you think I ran & pouted you'd be wrong! She just doesn't feel anything sexual anymore. It gives me hope that your decreased libido righted itself eventually but, in the mean time, we're stuck. While I still have urges I, too, have slowed down over the years...perhaps mercifully. When I was in my 20's the crack of dawn had to be careful around me!
Have any other women had a loss of libido to the extent my girlfriend is experiencing? By the way, if you're suspecting an affair, perish the thought. We're a good team & look forward to happily growing old(-er!) together.