I'm so sorry OP. There are no real answers, but here are a few thoughts, based on what I found helpful in my mom's last days and the months since:
- given the cognitive decline you describe she won't retain anger or bitterness at you for not being there at any given moment. But you will always know that you were there six days a week and spending tons of time on the phone w/ her daily. You are doing that and you need to give yourself credit now and in the future when she's on your mind.
- hospice personnel are amazing. They will almost certainly know when the end is truly imminent and will be able to give you warning. You should talk about this with them so they can tell you what to expect.
- You will not be able to make her end of life experience perfect. It's not possible and you can't control things. It will happen however it's going to happen. But no matter how it happens, you are giving your all.
- It would not change her outcome, or significantly alter the course of events, if you were to camp out at her bedside 24/7. It might not even help her or be a positive experience for her. But it would likely be impossible for you and your family and take a terrible toll. You have obligations to yourself, your husband, maybe kids/job/other commitments? that don't stop during this period. You have to "keep the train on the tracks".
- You can only manage her care if you are keeping your head above water. If you make yourself crazy w/ guilt or overextending yourself you might not be able to cope with whatever you need to manage when the time comes.
- if she's in hospice then you are ensuring she is comfortable, well cared for, and safe. You're doing an amazing job OP.
Don't beat yourself up. Death is hard. You are getting the gift of a slow, prolonged goodbye - which is affording you tons more time w/ your mom. That also brings with it guilt, stress, uncertainly, guilt, worry, guilt...

But you're doing a great job in an unwinable situation.
Think about what you would say to your best friend in a similar circumstance and extend the same grace to yourself. Hugs.