Anyone else found themselves in this strange and horrible limbo following parent's death, with regrets and second guessing constantly?

My lovely 88 year old Mother who lived with us died two weeks ago, rather suddenly and unexpectedly. She had been going down the regular "dementia hill" it seemed, incrementally showing increased signs, but still walking, talking, laughing, eating, not incontinent, but just plain slowing down. She began having heart issues (angina) again, and a blockage of some sort was discovered after a nuclear stress test. We opted not to have invasive procedures done, as the consensus was her dementia would spiral downwards as it did when she had a pacemaker, and another likelihood was she wouldn't survive the operation anyway. Long story short....she had bad pain Saturday night, went to ER, hospital kept her overnight, found she hadn't had a heart attack, but had bad chest wall pain. They felt a heart attack may be lurking to strike anytime. When we got her back home she was very sick from the morphine they gave her for pain. She dry heaved all night. The following day she was worn out, way more confused and disoriented, and fatigued. Things just kept going down hill. Her Dr. actually made a house call Wednesday evening and she perked up. He said she was fine, and just needed sleep. He prescribed 12.5 mg twice a day of Seraquel (sp) which he said would help her sleep. I gave her the one dose of it on Thursday morning. She slept and then would awaken fitfully, thrashing about, seemingly panicked. Myself, my husband, and family friends were with her every minute, calming her, soothing her, until she fell back into a sleep. This lasted about four hours, until she fell into what appeared a comfortable sleep. Two hours later she was dead. Ever since then I've been grappling with regret, guilt, extreme sadness, unbelievable restlessness. Why did it happen like this? What should I have done differently? I didn't even get time to say Goodbye. I thought she'd sleep like the Dr. said and awaken rested and back to herself. Now she's gone. I spent every day with her constantly. Now I'm filled with regrets of why didn't I do this, why didn't I do that, I never got her that new doll I was going to, I didn't tell her I loved her enough, and what a good Mom she was to me, etc etc etc. I feel as though I'll explode with sadness. Has anyone been through this type of thing, where it's like you could've, should've, done more. But it's too late? How long will this last, I wonder. I feel as though it will be with me forever. I took great care of her all those years, only to let her die in what seemed to be a frightening way....until the last hour or so. I didn't know. I was clueless. I feel like I let her down at the very end of her sweet precious life.