It's bullshit that baby groups weren't a thing. My mum definitely went to groups in local community centres and church halls when my youngest brother was a baby and toddler because I remember it and she has anedotes from conversations there.
I didn't do one each day but having a couple in the week made the difference.
I also let the housework slide in favour of a daily walk and something that got us out of the house. We went swimming once a week with a friend and her baby at our local leisure centre because it was cheap and tired the babies out. One baby group was near the town shops so I'd park by the hall and after the group would go and get the little things I could get in those shops or pop to the library.
It is so important for your baby's development, health and wellbeing that you are well mentally and physically. Also, baby's communication skills will develop better when you're singing and playing with him - which is what happens at these groups! It's good for him and you! It's great for you to build these bonds with other new parents, it's essential for reducing isolation, for sharing problems and building your village and your community.
I'm not a great housekeeper but what helped when my babies were little was having a shower at night rather than the morning, setting the breakfast table ready for the morning so I just needed to get the milk from the fridge. I stopped ironing.
Also, any day I'd stay in our flat it'd end up even messier because we were in it!
Your MIL is being judgemental and it hurts you. You know that you are doing the right thing for you and your baby. Ignore her. Your husband is mad if he can't see how exhausting and all-consuming a colicky baby is, and how lonely and stressful it would be to stay in the house all day AND how impossible it is to do any cleaning or tidying in that time!
Are you breastfeeding or are you able to leave the baby with your husband on their own for a few days in a row? One day won't be enough for him to realise the pressure you're under. Especially only 4 months after a traumatic and physically exhausting birth. You are still healing in body and mind.
Screw the "back in my day" crap. Just because she had a miserable maternity experience doesn't mean you have to.