You don't ever let go. He'll always be a part of you. I think what you maybe referring to is how to live each day without him physically being here.
My best friend in this world, Penny Rogers, died a few years ago (she was in her 50's). Penny is the sister I never had and I was beyond grief. There were times I thought I just couldn't make it through that day. I cried until I didn't have the strength to cry anymore. Somehow though the days, became a week, then a month, then 2 months, then a year. I found that I can tear up, then smile when some little thing (like a can of gravy in the supermarket named Miss Penny's gravy) reminds me of her. I'm crying as I'm writing this to you because it's a fresh rememberance that I miss her. It's Ok though, because I allowed myself to move at my pace and not what other's felt I should do, that I'm able to, as others say "move on".
I don't think people try to be callous, it's just that they're uncomfortable dealing with death and because they don't know what to do or say they feel that by now you should be "over it".
If you haven't been, please find a support group. There you'll find people in varying stages of their grief and the support you need.
Please know that just because you are living your life it does in no way shape or form negate the life of your son on this earth.
Many blessings,
E.