My advice would be stop trying so hard. Just relax and let yourselves 'be' with them (same advice for buggerboooo). It sometimes takes a while to build bonds with dogs anyway, especially when you have lost one so dear to you, but it will come in time. I know it's hard, but you do need to try not to make comparisons between the two. It can be very easy to look back with rose tinted spectacles when you have shared a dog's lifetime and tend to remember all the good times, but I bet there were days when your other dogs also had you counting to ten with their antics, at least while they were pups?
My current pup is dog number 7 for us. I have loved every single one, but very differently. Two of those dogs have been extra-special and the ones that will forever hold that extra piece of my heart that no-one else will ever get, but there's still an awful lot of heart left for me to love others and appreciate them for the individuals that they are.
When I lost my big lad I thought I would never love another dog, losing him genuinely broke my heart and I was sure I could never bond with another dog as I had with him. I still had two at home and at the time, one was a youngster that was driving me bonkers with her scatty/stubborn ways and antics - BUT - I lost her last year at the age of 14 and found that once again my heart was breaking. That very same annoying youngster that I felt I would never bond with turned out to be my forever girl.
In fact I have just been on another forum telling someone exactly how she drove me scatty for the first couple of years of her life, I even went as far as to tell dh that I didn't think we could/should keep her, yet she turned out to be the most awesome dog that I will genuinely miss every day for the rest of my life.
Give it time, you have her/his lifetime to spend together and share all those experiences that will help to bond you together. No, they will never be your other dog, but that doesn't mean you can't and/or won't love them for who they are, you just have to treat your relationship with them as something completely separate from your experiences with your other dog.
I know it's hard and do understand why, but please try not to be so hard on yourselves. You are still grieving, but that doesn't mean you don't/can't love at the same time and if you allow yourself to, it doesn't mean you are being disloyal either, your other dog wouldn't want you to be alone and unhappy or for your love of them to prevent you opening your heart to another. x