My Little Bichon Just Passed Away

My little Bichon 'ZOLA' just passed away. I know what everyone on here is now going through. It's unbearable...
He was 13 and 3 months and we had to put him to sleep yesterday at the Vet's. We had little choice as he couldn't breathe well, had diabetes and other issues.
Our only other option was to pay several thousand and put him in intensive care on a respirator for a day or two and hope he'd get through it.
Firstly we couldn't really afford such prices and even if we could the vet told us that it's no guarantee because of his other problems...
Our heart broke when we saw him trying to breathe in his cage at the emergency hospital.
Basically his little lungs were only operating at about 20% by then, so they needed a quick answer.
We just stood there and bawled, there was nothing really to do. We could have bought him some more time I suppose now with the respirator etc...and I'm presently beating myself up badly for not doing that but in the moment we just had to say to put him to rest.
The vet was so nice and caring and said you may buy him some more time but as he's old, these issues just won't go away at his age and you'd just have to face his passing sooner or later anyway. I'm currently going throught the 'if only' 'why didn't I' stage and it's extremely painful.
I'd give anything to have him back even for just another month but it's not possible. He was such a good little boy, an angel, as every person's pet is I'm sure. I loved caring for him everyday and he loved us and had such a great life of good food and love and sleep and play.
Someone please tell me it gets better and if I did the right thing in having him put to sleep!
At his age I guess I'd have to take the pain at some point, I mean, it's not as if he was going to reach 15 or 16 realistically...so my only (tiny) consolation is that he had a great run in reaching 13
we love you babi bello...I hope we can see you in heaven, I HOPE! I hope animals go there too...it would keep me going if I knew I could see him and hold his little body again and see his gorgeous face and his big black Bichon eyes
I hope someone reads this and responds. I need some help. Thank-you all for your time in advance
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Ciao little Zola