Let me preface this by saying there's what you morally may feel you should do, and then what you have to do. Your Dad is dead, promises made to him mean nothing except to you. If you think it matters it does, but it doesn't. Promising dead people things doesn't mean much.

Under the have to do, you don't have to do anything. You are a free person who can do what they want. You are under no requirement to spend time with anyone, be involved in anyone's life, or be responsible for them. You can choose who you want in your life, from your family, to some friends you consider family, to absolutely no one at all. You don't even have to give a reason, you can just decide to break off any relationship with anyone for any reason or none at all and you don't have to justify it or explain it to anyone if you choose not to.

As far as what you should do, that's a different story. You can decide to go with what "society" or "culture" feels you should do, such as spend all your time and resources on your mother, letting you suffer because that's a child's duty to some people, or you can explain your issues with her (probably won't work but lots on reddit think having long conversations with people will somehow change their opinion or fix things), or you can decrease your time with her and some other thing. You can internally decide what makes you feel moral, be that adhering to a non-promise that means nothing, or out of moral elitism that you need to support her because she may have her issues but you feel responsible, or something else you decide defines you as a person, whether that be caring child, or feeling indebted to a parent, to just deciding to keep her in your life and never trust her in any way nor put her in a position to violate that trust.

You make the call.