You cant forget him. You cant just put him out of your mind. There is so much truth in "it takes time" Dont try and get over this. Why are you trying to or wanting to? He was part of your life, a family member, like a child or any other member so think about him , remember him . Dont listen to people who tell you to "snap out of it etc" You cant and you don't have to.
One thing though Why did you put him to sleep? Was he sick, senile, not the guy he was? If so then you did the right thing well you might feel you did.
Give it all the time you need.Thats all the 'attitude adjustment" you need
Been there like you a few times. I still grieve.
Wandering aimlessly through FT since 1999.