How can it still hurt?

Ok so I am going to say sorry for this being so long first because I feel it necessary to tell you the story first. . . .

anyways, almost a year ago I had to put my dog down and he was my best friend, and he was only a puppy and the reason I had to put him down is because he had mental issues and he would bite people I loved and then not remember it and he bit my sister in the face and it was bad she had to get stitches and I was responsable and I put the mozzel on him and then put him outside and I kept looking at him and he would just look at me and he was wondering why he was being punished and I felt so bad about it but I I knew that I had to put him down because I have a little cousin that comes over a lot and I just kept thinking well what if this happened to her and I just kept thinking how bad I would feel if I kept him or gave him away and someone else got hurt so I decided by myself that I needed to put him down and at 16 that is a hard decision to make and when we went to put him down my mom was in worse shape than I was and I was walking him in the vet’s office and I guess his collar was loose because he pulled back because he knew something was wrong and he ran back to the car and sat there and just looked at me and was like telling me that he wanted to go home and I couldn’t take him home because he was a messed up dog and I knew that even though he never even dared to touch me and I was the only one he listened to and I just knealt down beside him and asked him why he was making this so hard and I put his collar back on and picked him up and carried him inside and when I got into the room, and yes I decided to be in the room with him because I thought well I at least owe him that, but it was really hard because he wouldn’t leave my arms and even when he was on the table he was climbing up on me and everyone just kept telling me that he loved me very much and it all just hurt so bad and the vet asked me to hold him so he would be calmer and then when it was done I just put my head on his little body and cried and I said I was sorry over and over again and I said that I loved him and just don’t know how to deal with it anymore because I still miss him sometimes because he was the one (and I know this sounds wierd but) that I would talk to when things just got crazy and he was the one that I would cry on and I just want to know if it will ever get better because I can’t even walk into the vet’s office without thinking of it and I can’t go into that room we were in I mean it hurts and it just tears me up inside. how do I make this feeling go away? please help me, I hate feeling this way and especially since it has almost been a year since it happened.