Sad, Confused And Guilty, did I let her go too soon? Options

[SallieMae's ...]

My SallieMae died in the very early morning 2/24/05. I had spent the whole day of the 23rd nursing her and praying she would get better. Sallie had had two surgeries in one month first a Mastcell tumor was removed from a muscle in her hip and then her spleen because she had a large growth on it, later we found that there was another growth both of which were negative for cancer. She came back from the surgery wonderfully and she was doing very well. I stayed with her everyday after her surgery all day to make sure she was safe. on the 10th day my husband and I left to go have some dinner when we got home she was limping. I figured she tried to get on my bed and too weak, she fell, we'll never know for sure. The following day she could hardly walk and would not take food or water and by mid day she did not walk at all and was writhing in pain we rushed her to the vet and they took xrays and found no broken bones, but some signs of infection. They sent us home with yet another round of antibiotics and pain killers. later that night she just laid there not able to sleep except a little and then she would jerk awake as if something had poked her really hard. I felt so helpless. I called the vet in the morning and he sent over a food supplement to see if she could hold it down, she did. she even wagged her tail and seemed okay. I managed to get her up and take a few steps she went potty and it was bloody. we were scared but, figured we'd wait until morning to take her in. I was able to feed her 2oz of this liquid supplement at a time so I figured I give her a little before we went to bed, she threw up and it was horrible, it was all blood. I called the emergency and they told me to wait until the AM to take her to the vet but I couldn't, I took her in. I think in my heart I knew that this was it. I didn't want to let her go. I was devastated when the doctor said she might get through the night but most likely will not survive this. They wanted me to leave her and I just couldn't leave her alone not if she might die and I would not be with her. I decided that she should be put to sleep so that I could be there with her in her last moments and that she wouldn't have to suffer another day like this one. She died very peacefully and I take some comfort in that, but I will never know what made her so sick and thats what is eating me up inside, I keep thinking if I had just stayed home that night I would have been able to do more for her. If I didn't force the supplement she would have healed, if I didn't get her up she would have made it through. I'm so wounded and in so much pain I miss my girl and I can't think straight. She was 14. We adopted her when she was seven she was abused for the first seven years of her life, from the day she stepped through our door, she was our best friend and our lives worked around sallie's health and happiness. she was our baby and we loved her deeply.
Today I went to the vet to return some equipment they loaned us and the Doctor made a comment that sometimes things seem more serious than the actually are and that it may have been something that could have been corrected. so theres the bug DID I DO THE RIGHT THING? Could I have had my Sallie a little longer? I don't understand why he didn't say that before I took her in to the ER. thanks for reading I need to get it out I am so sad right now.
Julie