Not sure if this will help but I felt exactly the same after DS2 and talking to other mums they felt exactly the same too.
I read somewhere that you should not worry about sharing your time equally but try and give your attention where it is needed at that moment and having confidence that it will even itself off in the end.
I really focused on how DS2 was DS1's baby brother and how DS1 was great at stopping DS2 crying and how DS1 was so special because he was the big brother. Also remember that DS2 won't care who is cuddling him at this age so give him to any visitor, relative, etc whenever you can.
I always tried though to make nappy changes those times when I gave DS2 the attention.
I can understand the sleeping arrangements but think that you should look to getting DH back with you and DS1 on his own in his room. Won't work every night but you need to feel connected to DH
Once DS2 gets a bit more interesting - smiling and watching DS1 then it will get easier. Once DS2 gets even older he will worship DS1 I am sure and when you see them having a lovely sibling relationship you will know that it was all worth it.
Don't forget the first 6 weeks are hard work with 1 DS let alone a toddler and a newborn. It will get easier!
If you are concerned about PND talk to someone in RL - HV, Dr, Midwife etc they will help I'm sure.