5y/o female passed away for no discernible reason

Good evening (in Australia) everyone.

I'm really just here to vent at a really terrible situation and I really don't have anywhere else to go.

Last night, our 5 year old girl passed away in her sleep.

She was a purely indoor kitty, with 4 siblings, including a terror of a best friend and brother.

She came out for dinner at 6pm, flirted with her big brother a little bit, got some headbutts, unicorned a pat from me, then went to have her post dinner nap on our bed. Nothing out of the ordinary.

At 7:30 my partner went in to get a sneaky pre-shower pat (she's a very timid kitty with everyone but me and had only recently started letting him pet her on the regular), and she was gone. At first he thought she was just in a really deep sleep, but it soon became evident that wasn't the case.

We rushed her to the vet, and they are of the opinion (which I had already come to after checking her over while sobbing my heart out) that she either had a stroke or heart failure. From her positioning they're 99.99% sure she didn't feel a thing.

I know that should give me some solace, but right now I just feel lost because I'd spent 5 years working on this abused little girl to get her to trust people, and when she was finally on the precipice, this happened.

Please don't torment yourself and feel so lost, you were that little girl's everything and you gave her so much more then you will ever know. She knew love and was secure for 5 years, to her that was all she ever wanted. She went to the Rainbow Bridge knowing that she will be never forgotten and that she is secure in a loving heart.  That she went peacefully in her sleep is a blessing, she should never have suffered ever again, and she didn't. It was most likely an undetected heart defect, and the fact that she was observed by you and a vet, and been found to have no visible symptoms, means that nothing could have predicted or prevented this. You gave her a family, and will be blessed for that and for opening your heart to these sweet little ones who so desperately need you.Go on to do good things in her name, she left you a legacy of love to pass on and to keep her memory alive. We will never understand why such a sweet little soul has to leave, but she grew to be secure in the knowledge that she was wanted and loved, and for her that was enough. My heart cries for the pain you are going through, but my heart celebrates that you have a partner and her 4 siblings to support each other and mourn her passing. She gave much to your family and you, even though the pain is unbearable right now, it is equal to how much you loved her and it is still better to have known her then never to have met her at all. Thank you for letting us know of this special girl, I'll pray for all of you, take care........RIP sweet little girl, you have left this world a little darker without you in it, but your star shines bright in the heavens from the warm laps of angels!