I guess it is too late for your mother now.
Maybe she had "adjustment" problems right from the start but some people simply cannot let things go because sometimes there are important principles at stake and justice and fair play to consider without which a person cannot find what is now fashionably called closure.
It takes two people to make a marriage contract and it is only right and proper that two people should end or dissolve that contract, that blame should be properly apportioned and recognised by both parties and suchlike. When a divorce is a unilateral thing, especially when the one who doesn't want a divorce is relatively blameless it can cause a deep sense of injustice that manifests as jealousy and hatred that can last forever.
At a legal level the lawyers and courts have a duty to preserve marriage at almost any cost and to bring about a reconciliation if possible.
These days divorce has become so routine and such an important contribution to the funding of the legal system nobody thinks about duty anymore. Blame is no longer apportioned or properly recognised and a divorce petition routinely contains so many lies, half-truths and exaggerations deliberately to force a court to grant a divorce it is bound to leave a very bad taste in the mouth of some that they might never recover.
Most misdemeanours within marriage aren't serious enough to persuade a court to grant a divorce yet it is years since I have heard of a judge tell the divorcing couple they are being silly and to go away and grow up!
There might be some political mileage if you tried to discover the history behind your mother's sense of injustice and resulting hate and jealousy and tried to understand it. Her attitude could equally be caused by a deep sense of shame, guilt or humiliation but in your case knowledge and understanding could be power.
Your first duty is not to your mother but to your kids and yourself and so it is time to stop being the child and become the adult in your relationship with your mother. Your job is to keep your kids away from negative or bad influences and so it is time to stop expecting or hoping your mother will behave in a particular way and instead tell her how things will be.
Tell her it is time she grew up and stopped treating other people as if they were her own private punch-bag.
Tell her you know why she is bitter and it is a shame she has allowed it to poison her own life but you aren't going to allow her bitterness to taint the lives of your family.
Tell her she has one chance to become a storybook grandma or she is going to be out, alienated, cut off, unwelcome, persona non grata...
Blood is not thicker than water and after your ultimatum the final decision will be hers and not yours. Your mother either wants to be a part of your family or she doesn't. I strongly suggest you judge her by her actions and have no regrets about your decision.
Good luck!