Sorry, about how You are feeling but believe Me when I say it is very normal.
Soon You will feel better .After You get Your new fur baby home,It will start to get better.My Mom passed away very young a few years ago.I had gotten Her a Chihuahua A few xmas's before She passed,Well that dog was so spoiled by Her that it had to have chicken Mcnuggets on Thursdays or it would get seriously depressed!
I inherited the dog back when Mom died and it turned out to be the only thing that got Me out of bed because I could not go on without My mom {My best friend}but I had to get up and feed it.Well Blue and I became very close and He passed 2 yrs ago and I buried Him at My Mom's feet,To rest with his Mom ! I had Him about 10 yrs and when He passed He took apart of What I had left of My Mom.
When I decided to get Daisy I felt the same as You-The guilt came in.But it has gotten better since I have Her.I feel happy again to take care of Her it is a real joy and I know Blue is looking down on Us from My Mom's lap and is happy for Me.
I hope this story helps You in some way.Relax and enjoy Your new baby.It's the right thing
