I was really moved after reading your words. I'm so sorry you have lost your grandson, I can't imagine what you & your daughter have been through. You sound a very caring grandmother & mum, I expect a lot of how you feel is also down to the fact that your daughter is Suffering too.
Life does seem so unfair sometimes especially when little ones are taken its impossible to make sense of it. I'm sure however short his life was, you loved every moment of being with him & his memory will always be a treasure to you both.
Bereavement counselling may not be the thing for you so you can always post on here, every one of us is going through losing someone very close & a lot of us are struggling with it. Sometimes it helps just to know that people understand how you feel & to know that you're normal. if you feel you need the depression tablets for now & your doctor is fine with that then don't worry. You need all the support you can get right now & there will come a time when you realise you no longer need them.
I expect his first birthday coming soon is going to be difficult but try to see it as a celebration of his precious life.
My mum died suddenly on New Years Eve & so far every day is a real struggle so although its not the same circumstances as yours, I do understand your sense of loss.
Your daughter will have another baby I'm sure & you will have something to look forward to again. I'm sure once his birthday has passed you may feel a little lighter.
In the meantime, you're in my thoughts, be kind to yourself & take care.