ahhhhh pedal (((((((hug))))))) it is the 2nd time this week i have had to hug for that reason. It hurts alot I understand totally. And the moments you have when you loose it are our way of dealing with the grief. It is way normal. It sounds like you gave him a full loving life!
I lost my favorite cat 5 years ago this week. It took me and my dh about a year to get over that. My guy was killed by wondering dogs. It has been seared into my brain and it still hurts bad. I wish I could have watched him grow old. He was 10 but still full of life! Good luck take deep breaths and allow yourself to cry. You will heal.
We have Bo now and if we had not taken him he's life would not be the same. He needed a good loving home. So I sometimes feel Niko's death opened the door for Bo. I would love to have Nkio by my side don't get me wrong. But Bo is very special too.