I remember feeling a little like that when my first was tiny. But the thing is, you find so much joy and wonder in the new, funny, wondrous, clever, curious people they develop into as they go through their lives that jt crowds out the feelings of nostalgia.
I just climbed a mountain (an easy one, but still a proper one and some difficult bits and stamina required) with my two boys. They're 10 and 6. I can honestly say that I have that same intense feeling of love and pride in their achievements and discoveries now as I did when they were tiny things.
Early parenting is an adjustment. It's the first time in your life that you've had such a close relationship with someone who changes so much more quickly than you do. So you can't conceive of loving the 10 year old in the same way as you love the 1 year old, because you don't have a point of reference for that kind of love for someone who changes so much so quickly.
But you will love and find joy and wonder in your future son every bit as much as the joy and wonder you feel now. These days with absolutely be precious memories. But you won't have lost your baby, you will have raised your son.