Not sure if this has been recommended already but Blue Cross have a pet bereavement service available:
If you are grieving for a pet, or facing loss, our free and confidential Pet Loss Support service is here for you by phone, email or webchat.
www.bluecross.org.uk
I lost my kitty of 18 years back in 2019, she had seen me through ALOT and it was devastating. I donated money to PDSA so she's got a plaque in their special pet memorial garden (
https://www.pdsa.org.uk/donate/donate-in-memory/remembering-pets), and I had her privately cremated and some of her ashes into a necklace because she'd literally been through so many huge life changes with me. I ended up adopting a new kitty a few months later, I feel that he was sent my way to help me (and me for him as he'd been given up SO many times in his short little life). It does get easier I promise, but grief is normal - they're such a huge part of life. I also really struggled initially with my new cat as he just was not my last cat, but once I accepted his ways and dealt with his behaviour I wouldn't change him for the world