Your loss is very new. It does get easier with time but I don't think the pain ever goes away--it just dulls and has relief through other loves in life. If I didn't have the others to hug & nuzzle me during these times, I'd go nuts.
Every person is different. For me, I grieve for a time, then I do not dwell on the passing, but the time spent living. It's the only way I can deal with it, espcially since I love the seniors so much.
I think of special "pictures" of those who have gone--Magic flying over the doorway, Solo chattering, Scooby with his paw hooked over his ear, SieSie knitting, Shadow running in the yard, Jodie grinning, Burp doing his 10 o clock snack stomp, etc. etc. Sometimes, the pictures bring tears, but usually a big smile and make my heart sing.
A sudden loss is a shock. In some ways hospice care prepares you. It also makes one aware that going quickly can be a blessing. It does not seem like it at the time, but it's something you realize down the road. It was that way with my mother and my father. They both went to sleep one day and didn't wake up. Unexpected, and a shock and also a blessing.
You don't ever have to get rid of his special things (unless maybe the food gets buggy ) if you don't want to. Hold on to them as long as you need to.
It's my personal belief that Harley is not that far from you and probably very close to you. You just can't see him anymore. But you will, one day.....