Baby Wonders

I've been thinking about this topic for quite a long time now. Or at least a couple of months. Jewell is now 6 months old. She just had her check up and it went splendidly. Her doctor had absolutely no worries with her development. I however, was not relieved after leaving the office.

My concerns:
Jewell doesn't roll over. She has only rolled over twice actually. And it's been a long time since. She doesn't like tummy time. People that see her keep telling me to give her MORE tummy time so she does become more active. She cries the entire time. I still make her do it every day, but it's extremely hard for me to watch. The doctor said to be grateful that she doesn't move yet, yet I'm still worried.
Jewell also doesn't really sit up. She only does it in the bumbo and it's not for very long. She screams when I try to practice with her. It makes me so sad.
I know I shouldn't compare, but it's really hard when you walk into day care each day and you see kids that were born on the same day or after her rolling around and giggling. Many have told me that every baby develops differently, I know that, but most of those people then tell me how early their kids did things, which is hard.

I'm not really asking for advice, mainly just kind of putting out my fears and worries. I think about it and pray about it all the time. I love my baby so much. I know there is nothing to worry about, but it still makes me wonder...