I'd guess it just takes more assurance. For years and years and years I kept trying to provide more than my wife wanted. I felt there was something I was doing wrong. If only I could discover what excited her she would love me more.
Eventually, after years of assurance and a little bit of counselling, I started trusting that what she was telling me was the full truth. I don't understand it. I don't have any idea how she can be happy with the limited attention I'm paying her, but I believe that she is, and it's taken a significant amount of pressure off.
Short version is keep assuring her, and give it time.
Me: 39 straight male in a V with
Wife: Kay - mono female - married 16 years
LDR girlfriend: Susan - poly female - 2.5 year relationship
Stakes - very intimate friend