I had terrible PND, and it took me literally years to get my head and life back to a 'normal' place after my first. Everyone was having second and third babies around me and I felt like a total failure.
Then, when he was 4, it suddenly all felt easier and we've now got a 5 year old and a six month old, and it has been just such a different, more joyful, and easier experience.
So it may be this way for you- suddenly you will just start to feel like you could do it all again- and don't assume it will be the same hardness.
I know you say that time isn't on your side, which I do appreciate, but don't put pressure on yourself to have the standard two year gap. I also felt like 'whats wrong with me' when I couldn't contemplate having another at that stage. But the more I started actually noticing people's family set ups, the more diverse they actually are. I know as many one-child families as I do two or three. My son has friends with siblings with a five or six year gap. I think the confirmation bias means that you always notice the thing you're looking out for (two under two type thing) but actually, in real life, families are much more diverse and everyone has different things going on.
So really- don't beat yourself up. There's nothing wrong with you. Being ready to have a second child sooner, or at all, doesn't make you more of a mother. Knowing yourself, your limits, and what will make you happy is a much, much better test of the kind of parent you are