A.H.
I totally understand you. When I was pregnant with my son my daughter was 1 year old. and she and I have alway had this clos bond. we are very affectionate in our family so she was always in my lap. well when I was pregnant I couldnt stand anyone touching me at all I had to move in with my mom becuase there dad was getting on my nerves(were not together anymore) well I just let my daughter sit on me and be close to me because I figured she's gonna rebel when the baby comes if I start pushing her away now and then baby will come and she'll take her anger out on the baby. they don't understand all they know is mommy loved them until baby got here. I say let him be with you. it might be annoying but it's only your hormones and he might take it out on the baby by hurting him/her when born.
As for your depression. I think your just not getting any grown up attention and your starting to feel isolated. get out and meet people. go to a mommy and me class or a free class at the hospital for pregnant mommies learing how to teach the toddler abouth the new baby. they have those and there are other mom's there and it's more of a social group that way you don't feel so alone. call "babies are beutiful". they can give you the listing of the classes.