I felt like that too, when The Creature died. We had to decant to PILs whilst building stuff was done to our new home. They have a massive house, and DH went into her room to see her one morning, and opened the window, whilst having a smoke. He forgot to close it. She only fell 2 storeys, but caught a stone planter, and died there and then. I was bloody hysterical.
I too doubted my ability to keep a cat safe, yet MCat and Little M are still with me. Like you, I thought long and hard before Little M came to live with us. He'll be 3 soon, had him since 13 weeks. MCat is 6 and was with DH from a kitten.
I have FlatCats mesh on my windows, and they have Loc8tors on their snap release collars (there are better cat locators now, with real time GPS). I lock the flap at night, and by then they're relaxing on their radiator beds. Also kept them indoors for at least 4 weeks, when moving with adult cats, obv. longer with a kitten - until after jabs and neutering, and time to recover.
It sounds as if you did everything you could to find her, Posie. I find it reassuring to know that my cats' insurance and chip details are up to date, as well as them wearing a tracker. But never on those elasticated collars, which can cause slow, painful deaths, only use the snap off ones.
Don't beat yourself up? There are so many cats needing homes.