Don't ever let anyone feel you have to grieve a certain way, or that there are acceptable and unacceptable "levels" of grieving.
You have lost a child, and she will always have a place in your heart. I'm sure it's just naivety on their part as I'm sure non of them have been through the same. You may have weeks at a time where your baby is all you can think about, and that's fine, you certainly don't need to "get over it".
Policing other peoples emotions is probably the biggest thing that frustrates me in life. They are your emotions to feel and you've had unique experiences to make you feel that way, they can't possibly decide for you that it's time to move on.
I've never lost a child, but i have experienced grief, and I dont think anyone ever really gets over it, but you learn that there are other aspects of life to be happy about, but grief and happiness are not mutually exclusive.
You're perfectly capable of carrying this grief and being a great mother, so nobody has the right to make you feel guilty for being upset about your other Child. I really hope you can rise above the way you've been treated, and feel however the hell you need to feel to cope with what you've been through.
Also thought I should mention that, while I'm sure you've been told, make sure you explore all avenues of support for you, as there are lots of organisations to help you with counselling etc if you do feel you should be coping better.
I really wish you all the best in life 