Am I making the right decision?

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Hi, I’ve never posted on a forum before but my head is all over the place (so please be kind!).

I’m 43 with a 6 year old with mild autism. My husband and I don’t live together but the three of us are still very much in each others’ lives and it works well. 

I’ve had four miscarriages in the past. My husband and I recently discussed going to a fertility clinic and trying IUI. We’ve both had tests in the past and are in good shape in that respect. 
I’ve now been diagnosed with antiphospholipid syndrome (basically sticky blood), which might have caused the losses and would be put on blood thinners. 

I’d love another baby, my son would be an amazing brother and would love a sibling. I also feel confident about raising another child with possible autism. 

I decided a couple of days ago though that having another most probably isn’t the best idea because I worry about not being able to give my current son as much attention and also the risks of an older pregnancy. My son doesn’t know about the miscarriages but I couldn’t hide a stillbirth!
I also don’t have much of a support network around me, although I am the type who always makes it work (and well). I work full time and can afford to buy in a bit of help if needed. 

I kind of felt though that ‘making it work’ likely wouldn’t be as beneficial as spending the time to teach my son to thrive, if that makes any sense at all. 

In my head this all makes so much sense, but I’ve imagined my life with two kids for so long and my heart definitely isn’t in agreement! 

I keep getting a feeling of panic that takes my breath away, like I’m making the worst decision of my life and time is running out to change my mind. I also feel so guilty that my son, who is the most loving, wonderful person (and doesn’t require a sibling to become a carer one day by the way) won’t have a sibling. 
I don’t know what to do and it’s breaking me in two!

I understand that you are going through a difficult decision-making process. It's important to remember that there is no right or wrong decision when it comes to having children. It's a personal choice that should be based on what's best for you and your family.
You mentioned that you worry about not being able to give your current son as much attention with another child in the picture. It's important to consider the practicalities of having another child and how it may impact your family dynamics. It's also worth noting that siblings can be a great support system for each other and can bring a lot of joy to a family.
On the other hand, you also mentioned the risks associated with an older pregnancy and the fact that you don't have a strong support network around you. These are valid concerns that should be taken into consideration when making your decision.
Ultimately, it's important to listen to your heart and make a decision that you feel is right for you and your family. It's okay to take some time to think it through and seek advice from trusted friends or family members. Whatever decision you make, know that you are doing what you believe is best for your family.

Thank you for such a kind and grounded response! I'm very grateful.
It turns out that my heart has won the battle and my husband and I are going to keep trying until the end of the year.

I've spent the last couple of days evaluating our lives, watching my son and trying to convince myself that it wouldn't work, but it's only served to confirm things the other way.
I won't bore you with the details, but our lifestyle (relaxed and fairly home/local based) won't mean that my son will be disadvantaged by a baby in the house. In fact, he'd love it. Any big days out are always with my husband, who will be on hand to help out.
I'm being super realistic and will still do my sums to make sure I can make things work financially, which I already know I can. This will include getting some help if need be.
The risks are also not as bad as I initially read, because your own genetics and 'biological age' come into play. I'd also be closely monitored because of my APS, which has nothing to do with age.
I know that I'll get some stick for being an older mum but I'm blessed with the maturity and open-mindedness to (mostly) shrug it off. I myself have older parents and I wouldn't want it any other way. Besides, I had to convince the babysitter today that I'm 43 as she thought I was in my thirties :)
I'm so grateful for the little person I already have that if it's God's plan for us not to have another I will be able to live with that and appreciate what I have. I simply want this too much to give up just yet and this whole thing has made me realise it.
Thanks again!