The OP knows her life, her family, and herself better than anyone on here. It really is up to her ultimately what she decides is the best.
With that said, a couple insights:
I am an only child and my mom had me when she was 40. She had dreamt of a large family because she herself is an only child, but she didn't get married until her late 30s and by the time she had me, her child-bearing years were pretty much over. She was actually regretting getting pregnant with me because she was running a 50/50 chance of having a child with extreme disabilities--in part because of her age and in part because of family history--and she didn't think it would be fair to me if I did end up having something. Of course that's something way out of her control, but when I was born and ended up spending my first weeks in the NICU when it seemed like I was on the losing end of the 50/50, she knew right then and there she didn't want to have that ordeal again.
I always wanted a sibling growing up, to the point I didn't have imaginary friends--I had an imaginary brother. My parents had just divorced and my mom had full custody of me, so when we moved to our new house no one we met knew our family dynamic. I still remember telling my neighbor that my "brother" lived with my dad at our old house and since I talked about what he "liked" so much, my neighbor bought me AND him a Christmas present that year, based on what she knew about me and what I had told her about my "brother." That must have been an awkward conversation my mom had to have with my neighbor to say that I didn't have a real brother...
My aunt and uncle also wanted a large family, but had an incredible difficult time getting pregnant. They eventually did with a girl, but unfortunately miscarried soon after. They waited a few years to try again, and were blessed with a boy...and then miscarried again. They wanted the third (or fourth) but they didn't want the heartache again.
13 years later they had a surprise baby girl, during their oldest daughter's senior year of high school and their son's freshman year. And unfortunately, they miscarried again within a year of having her. Now, their oldest graduated college, their middle is in college and their youngest is in first grade. It probably wouldn't have happened if they hadn't miscarried as much as they had, and although that's a form of heartbreak they had to endure three separate times, they couldn't be happier with their three and their age differences.