I delivered DS the day DD turned 16 months and all the feelings you are feeling are very normal.
You will find time with #1 with a new baby. Baby will be happily napping in the swing and you will be reading books, playing blocks or play doh with your first. The newborn phase is really hard (lack of sleep was horrible) but my first was so young and had no issues playing independently that it wasn't a problem at all! She hardly noticed him in the beginning and was very excited about the new toys I got her to play with (play kitchen!)
I learned to have everything you need on both floors. I had PnP down stairs, diapers for both kids, change of clothing for each kid downstairs. I learned to be very prepared cooking all her meals (and plate them) the night before because I always had a huge meltdown right when I was in the middle of bfing. All I had to do was throw it in the microwave.... I even had sippy cups poured and in the fridge because the last thing you want to do one-handed is pour a gallon of milk into a sippy cup!
Things get much easier quicker! Life was very manageable and still is when DS turned 6 months old and could play with toys sitting up or crawling.
DS was an accident (bfing, mini pil, no AF ever) but looking back if I had to do it over again I would have them close.