Sorry, Drazzil. From our various threads here over the years (including mine on my late 16 year old cat Molly in 2010), many of us here have been in your shoes and empathize.
In Molly's case I was haunted by declining a $1,000+ thyroid irradiation procedure back around maybe 2002 in favor of lifetime thyroid medication. Probably when she had her congestive heart failure that summer, if I'd taken care of the thyroid condition back then, we could've better treated her. I ended up spending over a grand on her emergency vet care in her final months and I beat myself up a long time about whether I shouldn't have had her euthanized before she reached the critical stage (her blood circulation basically shut down while I was at work on a Friday), why didn't I go for the irradiation treatment almost a decade earlier etc.
When it was clear the emergency vet could do nothing more, I made the final decision at an emergency vet about 4 a.m. (Sept. 11, 2010), and probably never cried that much as an adult in my life.
I did find it was really only fellow cat and dog owners who would listen to me and share their own experiences, and that helped me a lot then. All I can really say is, don't beat yourself up about it.
I went to a cat adoption event at a Petco a couple months later, but just found my heart wasn't in it. Finally around August 2011 I felt ready, and my mom - who cares for my cat when I'm on the occasional biz trip - tagged along with me to a cat shelter one time and then an Animal Welfare League. And JJ the tuxedo cat won the audition as really the only cat who seemed to bond with me. Now JJ is almost too bonded to me for his own good, but he's quite wonderful.
I still have a couple photos of Molly around, and she's never quite far from my thoughts.
Every pet owner's different though. Some friends who lost a pet were so devastated, they just didn't want to own one again. Others wanted to run out and adopt another pet the next week. And some like me felt they needed some time and space to grieve, before they felt they had it in their heart to adopt another pet. imho, all of those are valid responses to losing a beloved pet, and I'd never pass judgement on anyone's decision in that regard. You'll know what's right for you.