It will get better l promise,
I have all been there and survived you will too. I hated being off on maternity leave it was so lonely, l also hated being stuck inside in the winter, by time your baby is 4 months old the weather will be better, your routine will be more in place and you will be able to go outside, the cabin fever will disappear.
Just find your quick wins to help you keep get through the day
Mine were
My husband slept in the spare room as it was me who needed the comfort of my own bed, he was getting to sleep.
My husband made my lunch everyday and left it in the fridge so l could just eat it.
When l feed my dd my ds would sit by me and we would cuddle with a book or watch a tv programme.
I remember once l must of fallen asleep on the couch and l woke up with my sons blanket on me l said how did that get here he said you fell asleep mummy and l put the dummy in the babies mouth when she cried, he was 3 and a sweet heart.
You will be fine when you find your pattern and have a lovely time with both your children. Congratulations by the way.