HOT TAKE! You can't compare Glenn and Carl's deaths because they are wildly different, and Maggie's and Rick's way of grieving is different because of it.
NOW THIS IS MY OPINION. Don't start flaming me, but I started thinking about it because of the controversy of Daryl saying to Rick, 'You just gotta let [Carl] go.' and how people say that no one would say that to Maggie. BOTH GLENN AND CARL'S DEATHS WERE SAD, I cried so hard during Carl's deaths, and Glenn's was so devastating, but YOU NEED TO THINK ABOUT THE CIRCUMSTANCES.
Carl was bit, a freak accident (because he should've lived 😑), but he had time to say goodbye to his parents and sister, to write letters explaining what he wanted, and to accept his own demise. He died surrounded by those who loved him, feeling cared for until he pulled the trigger. And he died saving someone.
Glenn (and Abraham too) died violently, with a bat to the head, not knowing if their friends, partners, UNBORN CHILD in Glenn's case, were going to be okay. They didn't get to say goodbye, they didn't get any final words (ig Abraham did but that's not the point) and it wasn't in a caring, loving environment; it was with people with guns surrounding them, a mad man with a bat laughing and everyone crying.
Rick get's closure knowing that 1. the walker that bit Carl is dead, 2. He can strive to fulfill Carl's wishes, 3. That Carl died doing what he thought was right, and 4. That Carl would be happy knowing that Rick is trying to move on and progress, making Judith, Michonne and the rest of their family happy.
Maggie has to live with her husband/father of her child's murderer still being alive/now happy, consumed by trauma and rage that isn't properly managed. She will always remember her husband's bloody murder that happened right in front of her, as she tries to raise their son alone.
Now, Daryl never should've said that to Rick in the first place. Rick is allowed to grieve Carl and try to strive to what he wants for the future. But comparing Maggie and Rick (and Glenn and Carl) is stupid in this regard. Their emotions were both valid, but Maggie can't move on in the same way Rick could.