It's not unreasonable to have these fears. They're fairly common. Only you can decide if pregnancy and parenthood is worth the risk - remember that pregnancy raises the risk significantly of becoming disabled yourself.
I was disabled in-utero. I've known as long as I can remember that I wasn't what my family wanted - not only did some of them not like the audacity of being a girl, but I had further audacity to not be able to be a girl properly in their eyes because of my disabilities. I'm slow in many areas, I've needed mobility aids at times while also having been a very physical child, I fumbled my way through childhood and into adulthood in many ways. I was told very young that marriage is horrible, girls like me don't marry and should be glad of it, and the last thing a girl like me should do is have kids. I was told at a young age that kids like me are just burdens.
So I married young, and have four kids. When they were little, I used a mobility scooter to go out rather than a pushchair. I'd get asked my strangers how I do it, they saw a disabled mother of young kids, but to me, we were normal. While I have disabilities from before being born and a few that were acquired along the way, they're my normal.
And yes, I have disabled kids, though one prefers to call it difficulties in large part because he doesn't view his as bad as others in the family (he's been there when we've cared for family members with terminal illnesses - his idea of normal and ill health may be a bit skewed too). I wouldn't call any of them extremely so, but others may view them differently particularly when looking at them as toddlers/very young when very physical was a good description.
Couldn't cope and being devastated are completely different things
How so?
There are plenty of things we can't cope with that don't devastate us.
I can't cope in a loud office environment. In the office I work in, I can hear pretty much any conversation had at typical volume and if there are 5-6 going on at once, I can't cope, my brain fries, I can't hear my own thoughts to work, I will go find something to do out of the office for a bit.
I'm not devastated by people talking. I'm actually glad they're enjoying themselves, it's just not something I can cope with hearing at all once.
That said, yeah, parenting a very challenging child can be devastating. I don't think that solely means disabled children, not all of which fall into being very challenging. I've supported parents who have a child in a Crown Court dock - I can only imagine how devastated I would be to be their shoes. I had a brother in a similar situation (we weren't in the UK then so not a Crown Court, but similar level of severity) and living with that was bad enough. The only thing my parents ever said about it was my mother admitting that having to leave him the first time made her weep, that it broke her heart - and I never saw my mother cry, she was a turn sadness into anger kind of person. There are multiple ways parenting can be devastating - whether that's worth the risk is something only the person going into can decide, and I don't think any of us can ever know all the factors that might break us.