I've read that when we pass over, the very first to greet us are ALL of our animal companions we've ever known and loved so much. Jack is going to be the COOLEST cat in heaven now. He may even come back to visit you here on earth. Keep an eye on your other cats, they may notice when he comes around. My Peter kitty saw my Teeny who passed in Oct. As I was thinking of my Teeny, my Peter's head made such a slow, deliberate movement in a circle, and then stopped right in front of me. I said out loud, "Teeny, is that you?" And then I felt an emotional swelling throughout my body. When you're ready, try talking to him. It really does bring some comfort. Pay attention to your dreams too. Take care of yourself, and cry as much as you want. It's eventually going to be ok. If you need to talk, PM me.
Wishing you peace,
Rebecca