As soon as we start to care for anyone, or anything, there is pain waiting at the end.
That never stops us from investing our time and our love in our pets, and nor should it.
Of course you hurt, you would not be human if you didn't.
Intellectually,you know you did the right thing for your beloved pet. His suffering was only going to get worse, and keeping him suffering because you couldn't love him enough make it stop was never an option.
So you let him go, and it was, is, and will always be the right thing to do. You know that, it's why you did it. You cannot love a pet any more than to part with them when it is best for them, ho wver much it hurst you.
Emotionally, none of that matters at all. You are in terrible pain, and no amount of reasoning and knowing is going to stop that. You have to grieve for him, and let the pain come, so that it will go.
Eventually, when this agony lessens, you will still be left with the fact that you did the right thing for him. That will be the rock, revleaed when the tide of pain has gone out, and only laps at your thoughts occasionally in the future.
To replace that pain will be the good times you shared with him, and the courage you used to let him go gently and peacefully - they will remian, and that is what you just look forward to now.
Keep your memories of him as bright as they are now, and the sadness that infects them will heal and drop away, leaving them pure and clear for ever.
You will never get over him, or forget him, that is what loving a pet is all about, and you should invest your thoughts in the good times when the time is right, but allow yourself to grieve for him, that is right too.
The pain will pass - as pain does - the memories will not.