It is a totally normal worry - I had the weirdest, most stressful dreams when pregnant with dc2, all involving not being able to save dc1 from a horrible fate (usually falling down into a bottomless pit or being swept away in a tsunami/ shipwreck situation) because I had the newborn in my arms and there was nowhere to put him down safely without him also falling to his death...
Horrible.
Also groundless obviously.
I do think the guilt over changing your existing child's world utterly and no longer having as much focus in your one and only is normal.
However rationally I don't actually think it's good for anyone involved to be too intensely 100% focussed on one child, and healthier for the child not to always be the centre of attention!
So the guilt is irrational, but normal.
New babies need to be fed, warm, dry and physically close to a parent but toddlers can still be the focus of your mental energy, and if you stick to that your dc1 will not miss out.
Buy his present in advance including a little present "from" the baby, be sure there'll be a tree and pay attention to him, don't palm him off more than you can help and have DH bring him with him to visit and to collect you from hospital - do not have grandparents return him to find you and the newborn in a Madonna and child tableu in the living room, or nobody could blame him for feeling replaced!
As long as you handle it thoughtfully a new sibling is a good thing which will enrich his life, but your feelings are normal.