From my experience, the birth of a second child, can have you feeling overwhelmed, and tired. When your body gets so tired, the sad feelings can feel magnified. My kids are 7 yrs apart, and after my 2nd child was born, I was so exhausted and delirious for about 3 months. I had to call in help from neighbors and family so that I could get things done, and get some naps in. I was breastfeeding exclusively, which is particularly hard. My hats off to you, with a 23 month old, and a new born, you really do have your hands full. I think my best advice is to recruit as much help as possible, set a schedule, and also to try and find time for just you. I know its really hard to just have some mommy alone time, but that really helps, and might help you to feel better. The feeling of being overwhelmed can really trigger sad feelings, I felt especially inadequate after my 2nd son was born, I thought to myself that I used to be supermom, I can do everything by myself, and had everything under control. Its really really hard to ask for help when you have that idea in your head, but for me, it was either ask for help, or lose my mind.
As far as your husband is concerned, after the 2nd baby is born, finding time for just the two of you becomes almost non-existant. One of the kids needs you or both of you around the clock, and sleeping schedules are crazy, so alone time just doesn't fit. However, if you are able to schedule a date night either once a week or once every two weeks, even if its just for an hour or two, this will help a lot. My husband felt very much left out after my 2nd son was born. Having a standing date night really helped.
Once you get into your groove, and when the kids get a little older, it does get easier, and not so hectic.
Best of luck, and this is a great community of mommies that are here to help!