You're not a shit mum at all- the fact that you're worrying about these things at all shows you're a good mum. I think there's so much pressure put onto mums to go to baby sensory classes etc that it actually ends up causing a lot of anxiety. To me it's more important that your babies feel loved and cared for. I only have one and I struggle so I'm not surprised you're struggling with 2! X
Baby don't care about baby sensory class at this age. Those are meant for the mother who wants to get out and be in different environments or meet other people. I would rather stay at home all day if I only have the baby. The only reason I try to get out is because my 3 year old needs to be physically active, but to be honest it's freaking hard to juggle both outside. On the days my oldest goes to the nursery, I just stay at home with the baby.
I only take my baby to baby sensory group once a week of that makes you feel a bit better, all the things I said there are the reasons why except I don’t have another kid in the mix!