We ALL have days like that. It doesn't feel good, but it's totally normal. As long as they're few and far between, our kids aren't going to remember the bad days, or the bad moments that happen in pretty much every day. He'll remember the singing to sleep much more, I'm sure, and the feeling that he knows his mom loves him.
Try not to feel too guilty about these being his last days as an only child. Yes, things are going to change for all of you. Yes, you will have rough patches and sad moments, and angry moments too. (Wait till your older child hurts the younger one for the first time... Oh, the anger you'll feel!) He will have times where he acts out. There will be times you feel torn between the needs and wants of both of your kids. It's not easy. Take time to adjust and go easy on yourself and him.
The good side is that you're giving him the gift of a sibling. He'll be a big brother, and have someone to share life with. Maybe even a best friend. Kids are flexible in surprising ways sometimes, and he's still pretty young. It won't be long before he likely won't remember life before the baby was there. Once they can play together, it really does make life easier. And even before then, he'll likely be fascinated and in love with that baby and want to help you out.
There will be ups and downs. But it is so worth it!
I think the biggest thing I would like to change if I could go back would be to remember just how young my older son was when his brother was a baby. I think I maybe expected too much of him when it came to behavior and those bad moments that happened. I regret that. He was still practically a baby himself! (Not even two-and-a-half yet.) But patience can run thin and there's no such thing as a perfect parent. I would also have spent more one-on-one time with each of them instead of always being together, when obviously the attention has to go first to the baby's needs.