Abz, I went through the same thing with our Zoe. She was a shepherd mix, we used to joke she was a shepherd-demon mix. She was so cute, funny, smart, and such a big personality. Now that she's gone, I realize how much she filled the house with her personality. She had something wrong, though, that despite all the love and support we gave her, she still would bite us and the other dogs. She never bit anyone outside the family, but it was always a worry. Our vet and a trainer told us to put her down two years ago, but I tried to find another way, and hoped that maybe she'd mellow with age. Sadly, she only got worse and we had to make that horrible decision to let her go. She was terrifying when she would charge at us. She had to wear a muzzle and harness so I could protect us.
I really loved her, and I know she loved me because we did have some good moments, especially when I walked her. I wish I could have just been able to speak to her for five minutes for her to tell me what the problem was, so I could fix it. I do feel horribly guilty, because she was only 6 1/2, physically healthy and strong, but in the end I had to put the safety of my family, which includes the other dogs. It was an abusive relationship, and I probably let it go on too long at the expense of my other dogs and my family, because I loved Zoe that much.
I can't tell you how to resolve the guilt, but it helps to talk with others here as we share the experiences, grief, and guilt. I put up a photo of Zoe "smiling" and another one of her snarling, so I remember both sides of her, but I did love her despite that.