hiya sam,
I was just looking through the forums and came across your post, i don't know that i can be much help as i have never had to deal with your situation. I'm really sorry to hear your relationship isn't working it must be really hard especialy with a 4 month old baby. But i want to tell you that you have in no way let your son down, sometimes no matter how hard you try it just doesn't work. To be honest if you and your other half arn't getting on then it's prob best to separate as if your are arguing and there is tension your son will pick up on it which is unfair!
I hope you have support from family and friends? Have you tried some sort of help from professionals? that may help. If you really don't want to part ways then it could be an idea?
Big hugs to you and your little one.
Danielle xxx