Some people are blessed with a very compatible relationship with one or both of their parents, but most people struggle with those relationships to one degree or another. You mention several important elements affecting your relationship with your mother. These include her history as a single Mom who had to be both breadwinner and parent, her way of responding to the negative events in her life, and your desire to get along with her and make her last years of life better.
As a mature adult yourself, it sounds like you really understand how hard it must have been for her and why that bitterness might have developed. However, her continuing focus on the pain of the past clearly makes it hard to be around her. Her negativity might be the result of untreated depression, negative role models in her own original family, or long-standing personality characteristics. These are unlikely to change without therapy and are likely to change only if she wants to get help. It is not your job to change her and you really don’t have the power to do that anyway.
It isn’t clear that she sees her response to life as a problem. If she doesn’t see it as a problem, your options are limited. You might try talking to her about how you would like your relationship to be and what gets in the way of that. Try to be clear but gentle and non-blaming. It is also OK to suggest counseling as an option. You can do that a few times, but if she is not interested don’t keep suggesting it.
If she is not open to getting help, the best option is to accept her as she is. How do you do that? It may help to think about the things your own children are frustrated with in you and how you would like them to handle accepting you. Transfer this understanding to your dealings with your own mother. You might also need to limit your interactions with her to briefer or less frequent interactions. The goal of this approach is to stay connected with your mother, but to limit the frustration of dealing with her to a manageable level for you. This will also help you to better accept her. Our parents are very dear to most of us and we wish we could heal their wounds. This would truly be a wonderful world if this were within our power.