Saying Goodbye to My Best Friend

I'm about to have an extremely hard time in about a 15 minutes or so. My English Spaniel, Bandit, is being euthanized between 2-3 today. He would be 14 in December... but he's been going downhill for the last several months. Constantly circling, whining in pain, has a hard time lying down or standing up, basically slides to lying down when he goes to eat. We've been talking about doing this for him for about a month but every time we went to schedule them to come over for it he would suddenly act young like there wasn't any problem. But he just keeps getting worse. Today is one of his good days.... I gave him many treats, brought him to his favorite pond and park, a car ride, letting him rest now. Guilt and grief has been crashing through me for so long already. I'm his person and he's been my boy. He's the last of the origional 3 I had most of my life sonl far. He's one of the reasons (other than financially) that I'm still living with my parents. My eyes already hurt, my head hurts, I feel like I have no tears left to shed but I know I'll have more when it's time. But God even though I know I've given him such a good life I feel like there's so much more I could have done for him, should have done. He's been in my life 12, 13 years. I can hardly imagine being without him and it feels too soon. 😔💔 I love him so much