What a horrible situation to be in, I quite understand why you would be crying about it 
Take your time, let the emotions settle a bit. The think through what your concerns are about FASD specifically if that is the main issue here.
Unfortunately many (most?) children placed for adoption will have a mother who drank alcohol/used drugs to some extent. The degree to which any child is/ isn't affected will vary hugely.
Like any 'condition' the support sites will be full of the worse cases because they need the most support. I am a member of several FASD Facebook pages, but rarely post (except to encourage and support) because DD is minimally affected and incredibly 'easy' compared to what some others face. No-one knows how much her mother drank during pregnancy, but she was certainly drinking every day soon after and died of alcohol-related disease in her 30s (was mid-20s when DD was born). So DD was very high risk.
However, she's cognitively OK, difficulties with impulse control, emotional regulation and focus/attention, but nothing that is too problematic at home. Problems at school because they are rigid and old fashioned and have got her into such a state of anxiety - but that's another story!
Every prospect (at 11) of staying in mainstream schooling, getting some qualifications, living independently, having positive friendships, holding down a job and relationship in the future.
Just as importantly, lovely to be with - loving, funny, relaxed, interesting (why is her favourite word!), definitely quirky but in a way that is very lovable (not just by her Mum!)
A 7 year age gap is a good one - your birth child will be heading for more teen independence and will be physically much bigger and stronger. But of course, if there are major problems they will be affected by having less of your time and energy.
The trouble with FASD is you just don't know how it will go. So yes, it is a risk. Only you can say whether you can accept the risk in this particular case.
Take your time and don't be pressured, do what is right for you. No-one here will judge either way 