Poor you, it does sound a tough time.
Many, many mothers go through something like this (whether full-blown PND or not), not always straight after birth but often some months down the line. The tiredness, the relentlessness of it all, and the fact that there seem so few rewards for so long.
Some babies are just harder work than others (whether sn or not) - my dsis had a really, really tough first year with her dd (screamed constantly, was never happy) but it suddenly changed at a year old and they are both happier than ever now.
It will almost certainly be more compounded when your baby has sn, but it will get much, much easier. This weather doesn't help, when you can't get out so much, but when it gets warmer, and he can get out of his pram and be on the move, I am sure he will be happier.
In the meantime try not to think too much about the future - there is no way to tell what will happen but it won't be harder than it is now, for either of you.
Keep posting here, everyone here knows some of what you feel, and you will find lots of support and understanding.