I’m sorry for your loss- it sounds like you and Gus had a strong bond.
From reading your post, I hear that you are sad about the loss of your beloved dog, and miss him dearly. It also seems like you feel conflicted about your decision to put him down- you feel bad for doing so, but you also think it would not have been best to prolong his suffering. It is normal to feel this way, and the way you feel is valid.
Your post got me thinking about this analogy- grief is a ball that sits in a box with a pain button. At the start, the ball (grief) constantly presses against the pain button, and the pain can feel overwhelming and difficult. However, over time, the ball shrinks, and hits the pain button less often, making it easier to function. However, it can still hurt from time to time, and some days might be more difficult than others.The ball can take time to shrink, and there’s no set timetable for coping with loss. Take your time to process your emotions, and allow yourself to feel them.
It might also be helpful to find ways to continue the bond with your pet while acknowledging that he is gone. https://griefline.org.au/resources/understanding-the-symptoms-of-grief/#Continue_the_Bond_with_your_loved_one_while_coming_to_accept_that_they_have_gone has some tips that might be beneficial.
I just want to end this reply by acknowledging your strength in reaching out on the forums. Keep reaching out to us if you need someone to talk to- we are here to listen. You can also call the helpline at 1300 845 745 from 6am-12am AEDT if you would like to chat with someone.
All the best.