Am so sorry for your loss.
Firstly, please just try to hold on to the fact that you did everything for him. You sought treatment and then, sadly, had to make the decision. Although hard, that decision you took is the kindest and most loving act you could give. It hurts you, but it means that he isn’t suffering.
Secondly, I and others, completely understand that feeling of “please reassure me it gets better”. When you lose a beloved pet, it can feel so painful that you just can’t imagine ever feeling ok about it. All I can say is - it WILL get better. But it will get better in your own time, everyone has their own timeframe to when that is.
Don’t beat yourself up with time limits etc. Let yourself feel sad when you need to, let yourself smile when you think of him fondly.
“Give yourself time”, for me, is one of the most maddening phrases. You want to shout at someone who utters it when it’s raw for you. Yet, when you’ve gone through your personal allowance of time, you realise it’s true.
Not sure if you’re aware, but there is a board on here called The Litter Tray. You may want to post a thread in there for further support, etc. They are a lovely bunch and you can take your time to post as and when you feel you need to.