Hi. I know how it feels to be put on the back burner when it suits the other person, believe me. Friendships are often unequal and maybe one needs the other more than the other due to circumstances.
Friendship is also interpreted in different ways, in the same way marriage means something different for everyone. She needs your support, and you need her because you haven't a family.
So your friendship is unbalanced at times, and you have to listen to moaning which makes you anxious and uncomfortable. So is it enough to break the friendship?
You also don't have to listen to moaning when it goes on...interrupt and ask for some positive news, make a joke of it. Just don't let it go on...change the subject or say you have someone at the door or are about to go out. Take control.
When your friend is busy with others, take the opportunity to do something different, to fill the time with something you enjoy or haven't done before...go for more walks, visit a place you haven't been before, go to the library, watch a film, do some baking. In other words keep busy, distract yourself.
I know if I had my sons/grandchildren round here and hadn't seen them for a bit I would also need to concentrate on them and not do the usual routine phone call. A friend would understand and not take it personally and I would do the same. Give and take. If you're giving more than you are comfortable with maybe you should have a talk to her.
Maybe your expectations are too high and that can cause resentment and disappointment. Can you lower your expectations? Just some suggestions. I hope it works out.