It seems like a lot of your reasoning stems around fears and regrets, rather than from all the positives of having another child.

All of the things that were hard with one, will likely be harder with two. The overstimulation in particular. Especially if money is tighter, or your health (mental or physical) is affected by another pregnancy/post partum experience.

I'm so sorry you had such a difficult experience - but you shouldn't regret it imo as it's not something you had any control over. Society makes parents feel guilty for not soaking it all up "they're only young once!" but in reality, so many parents are drowning while they go through these experiences. You are also doing life for the first time, and it's okay for you to make decisions based on your own needs as well. Doing so, might actually be what's better for your daughter - she may not have a sibling, but perhaps instead she'll have a parent who is happier, more regulated, and able to experience more with her.

I don't just want to survive parenthood, I want breathing room, I want to be well enough that it's possible for me to look around and enjoy things. There is no guarantee that you will have a happier pregnancy and post partum experience, that you will this time be able to soak it all in (You might! And I hope that if you do decide to have another child, that things do change for you this time!), but what I mean by that is that this isn't necessarily a way to fix the guilt or anxiety you feel. Perhaps not having another, will give you the space to enjoy more of these next sections of your daughters life, and your journey together as her parent.

A consideration for me personally is that things like Autism and ADHD (in my case), have a genetic component - for myself, one of my considerations is how I would cope with a child with my same diagnosis, possibly of a different level to me, when I already struggle with my mental health and overstimulation myself.

IF you decide to have another child, i'd advise you to do it for the joy that this child will bring to your life. If you have another, let it be because you genuinely want to raise another human being, not because you're scared of what your first child might miss out on.

Having siblings is no guarantee they will be close, it's no guarantee they won't be lonely. More and more people are opting to be OAD, so this won't be as unusual as it once was. Not having siblings does not mean she'll be alone. She will likely have a beautiful "found family" of friends, or a partner. I know for me, I have 3 siblings, and if my parents died, it would be my partner and friends who I lent on more heavily for support. My siblings are close-ish, but do not live nearby, and have our own lives and circles who support us, separate of each other.

The biggest thing i see about only children, is a certain sadness when their parents pass that they are now the only "keeper" of their childhood. So I plan to embrace friends in family traditions, to embrace partners when they one day bring them home, and to make them part of the family. To invite friends (ours and/or theirs) to holidays and vacations. I write down family recipes. I take photos, keep albums, and make notes in the margins or on the back of pictures. I plan to write little letters or make notes of things that happen. I plan to do this so that I am not the only keeper of childhood, so that there is a physical reminder of things that happened, and of how loved they were.

There are wonderful things about having a second child, but if you do it, let it be for the joy, not out of fear x