What do I do about this longtime friendship?

I have a friend I met at work about 18 years ago. We collaborated and commiserated at work and became really close friends who shared information about our lives outside of work too.  We would get together and go shopping, go for pedicures, and go to dinner. After about five years, we both stopped working in the same office, but we continued to email and text each other both during the work day and afterwards, and we’d still get together pretty regularly. After a few more years, the get-togethers decreased in frequency, but the emails and texts continued. Now I’m retired, and she’s still working, so the emails have pretty much stopped, but the texts have continued. As for get-togethers, they’ve been reduced to getting together for dinner once a month.

We share a similar sense of humor and have laughed a lot together, and we share some other quirks and personality traits that have made us compatible friends. But in other ways, we’re very different people. She and her first husband had two children together, and she went through a long, challenging separation and divorce with him, which I helped her through. She’s now remarried, but this second marriage is also a challenging relationship. I’m in a stable, happy marriage and have no children. She also has a lot of health issues that cause her pain and frustration, and for most of the time I’ve known her, money was always a concern. I’m blessed with good health and strong finances, such that my husband and I were able to take an early retirement. Perhaps for all these reasons, or perhaps because we’re just wired differently, she has always been more of a negative person, and I have always been more positive. Over the years I’ve talked with her about her marriages, her health, her finances, and her self-esteem, always trying to give her advice and encouragement. I’ve tried to be there for her, and she has always been very appreciative and grateful for my friendship. And there have been a few times when I’ve shared concerns and situations with her, and I’ve gotten advice and encouragement from her too. But the times she has needed support far outweigh the times I’ve needed it.

I understand that good relationships do not involve keeping tallies or giving and receiving equally. I understand that people need different things from relationships at different times. But in this relationship, she has always needed help and support from me more than I have needed from her. And she always has told me she feels bad about that, but I always tell her it’s OK. She is seeing a therapist now, so at least she does have one other person she can talk to, which is a relief to me, because sometimes I’m not really sure I’m giving her the best advice and support.

After all these years of being the one person she can always talk to, complain to, ask for help from, etc., I’m kind of ashamed to say that I’m getting tired of it. There’s always something bad going on in her life that she wants to tell me about, and I always feel like I have to provide a kind and supportive response. And I don’t feel like I can say, “I’m tired of this dynamic,” because I’m her closest friend, and she will just feel worse.

I do still like her, but I feel trapped in this relationship and don’t know what do to.