Thank you for your post and sharing your anxiety. I'm not sure if I have any 'tips' for you but I will try and be helpful anyway.
I read your post a few times and I wanted to ask about it - the title of your post is anxiety about the future, and yet at the end you talked about not wanting kids and your grandma not happy about that decision. Where about's does the anxiety come in here? What are the things you feel anxious about?
I can relate to your post in certain ways, as I've had my life planned out by other people and my life didn't fit the plans. For me, it was a job and a marriage. I think there's this wonderful idea in people's heads about wanting the best for you and having an idea and a vision of what the best for you 'looks like'. So when anything disrupts that vision, it can be really unsettling. So I'm imagining your grandma cares so much but is so unsettled by it which is why she keeps bringing it up.
I wonder if maybe you can see this as a way of her caring and coping, but being able to own your decisions at the same time. You don't have to question them if this feels right for you- only to trust in them.