Pet loss

My beautiful Pit bull Hadassa, passed away tonight... My dad and I let her out in the front where we have our pool and most of our garden open. She wasn't feeling well so i decided to let her out for abit as she doesn't come out this side very often and is usually very happy when we let her. I keep blaming myself if only i had checked on her a little sooner and if i had not let her out maybe she would be okay... She drowned in our pool and as i jumped in i kept thinking she would be okay, but it was too late. I feel horrible and can't help to think there was something i could have done or how i could have spent more time with her. My eldest dog had to be put down last month and now this. I wish this feeling could go away. I wish i could have done something, she deserved better than that and I'm sorry.