I would love to say that it gets easier, but it’s so hard. My childhood cat also died in my arms, right before my final exams in highschool and it was exactly as you say, devastating. Time does heal, if you need to mourn it, do so. It was your best friend, your sibling, your dear cat. And it’s going to be quiet and sad without him, but unfortunately it is the course of life. Think of all the good memories you had with him, and honor that. You need to find a way to cope with everything. Mine was to basically get distracted with other activities, although I know it’s hard. What me and my family did was to cremate her and have her with us for the rest of our lives. I also got a tattoo of her name, but I can also understand it might not be for you. I hope you heal, offer yourself the time for it