I had tried various mothers group and felt exactly the same as you. It is really hard when you are among people yet feel so lonely. I ended up giving up. It wasn't until I was diagnosed with PND and joined a support group that we are now friends and we see eachother every few weeks. I think because we all opened up to each other and they were genuinely lovely ladies.

Some playgroups are for babies, you could give that a go. Keep trying until you click with people you like. Perhaps you have a local community centre that holds play groups. Apart from that you could start up a chat with women you see with babies the same age as yours and suggest a coffee. Best of luck