OP, on my birth board there were loads and loads of people who split.
I come from a background where talking about splitting isn't the norm at all (and on my birth board it was totally the norm). Only a last resort (unless abuse etc of course).
I remember the time oh so well. You're constantly feeding the baby and up all night, for the entire first year it is hard. 4 months has that whole leap thing. But to be honest it's the constant looking after, home chores and not having any time just being a couple that you were used to for years, that is hard to adjust to, even if you're not actually thinking about it.
Bickering constantly is the new norm and ends up being really frustrating. Gosh, DH and I were super duper close, weren't the type to argue etc but this new constant bickering was a shock.
I would say it wasn't until our baby was 1 year and a couple of months that things got back to normal, or a new normal, more patience etc. But leading up to it, there were many (note the many) times where I'd feel upset and then we'd sit and talk about it, realising that I'm stuck in one place feeding and DH was working, trying to help when he came back with washing baby clothes, cooking, cleaning etc that we hardly spent time together to connect. You can connect with a baby too, but when you're barely in the same room then there's hardly any time to.
We had no family support (I moved away which made it hard as mine are hands on with our kids). So it was constant getting used to a baby and exhaustion for the whole year.
The aim is to set time together when baby is sleep and just talk about it. Talk talk and talk. Calmly talk. Realise what the other is going through. Understand that things will get better once baby is more independent and it's just about riding it out. It can honestly work. A lot just give up though. Not talking about the ones where the man goes drinking or out with his mates all the time leaving the mother to do everything.
Just keep reminding each other that there's a choice: accept everything in your stride and take each day at a time, or be miserable and not actually stop and think of how things now change after kids and it's just working towards a new way to handle everything.
As a side note, I bf'd my DD till one and a half years. I remember my DH getting more and more encouraged about it as the months went by and seeing how did never got sick when everyone else's babies were (apart from the once a year cold we get). How healthy she was by mummy's milk etc. That really helped me through the difficult times too and he still looks back at those memories that despite how hard it is to sit and feed feed feed, we persevered together but he gives the credit to me. I never knew I'd ever bf to be honest, second baby was easy peasy. Props to your for ebf though! That is tough.