Congratulations on your beautiful baby 
It's very understandable to be anxious about the future when your baby is three months old.
I hear you loud and clear op!
I had my only child when I was 39. I wish I could have had more but it wasn't medically possible.
It's swings and roundabouts.
Positives: I had had a career and some life experience and travelled widely before becoming a parent and so didn't feel trapped by the early years. Dh and I were comfortable financially. I think being older made me a much more patient and pragmatic parent during the sleep deprived drudge years and I could afford to be at home with her quite a bit and I feel we really enjoyed it on the whole and made the most of it. I really would have made a crap parent when I was young frankly, as I was very insecure and lacking in confidence. That is why I genuinely admire young parents who have got their shit together.
We're not rich but we've been able to give our child a good lifestyle with opportunities to do lots of extra-cirricular activities and freedom to take holidays etc but generally have a settled and secure (as much as life is ever secure!) existence.
Negatives: my parents were too old to enjoy my child much or actively play with them but I have older siblings who have taken on that role. Between just before their birth, and up until their eighth birthday, my child lost all four of their grandparents. 
.And some of the grandparents were very ill and one had dementia which put a strain on me and dh.
I was one of the oldest mother's at the school gates and have been mistaken for "granny" once or twice. And I'm not going to lie but the teen years clashing with menopause can be hellish at times. Your heart sinks when they announce they are going out to a concert and you are required for taxi service at 1am on a Friday night. I simply don't have as much energy nowadays. I'm less patient and funnily enough, I feel a bit confined by chores and cooking set meals for the first time ever and I'm ready for things to change when they leave home (sounds awful doesn't it)? Also , it's a bit of a blow when you realise that they don't remember much about the full class parties you threw every year when you stayed up until 3am to ice the cake and prep the gift bags; they only remember the tensions and (as they view it) the injustices of adolescence (hoping this will evolve more favourably later on).
Overall though, I feel so blessed to have had the experience of motherhood and I am aware that many people do not.
Having a child has broadened my horizons so much (we live abroad and so have discovered the culture of another country through my child attending state education and learning the language) .
I'd love my child to have had siblings, but from a very selfish point of view, yoy only need one child to experience the delights of bathing a baby and to be committed to the sometimes not-so -delightful nine years or so of endless school runs (before they are old enough to take themselves) and play dates etc. And to come to understand yourself better through seeing yourself mirrored in their eyes (sometimes in a good way, and sometimes reflecting your own faults). And best of all, beng able to track the development of a truly unique individual, who you love completely, and whom you would happily throw yourself under a bus for, any day of the week!
Those are the important things to focus on. The fact that you get to experience all of that. Not when you experience it, but that you have the opportunity to experience it and treasure it!