LRDtheFeministDragon I am glad the thread is making you feel better.
You've asked a few times how not to dwell on things. I think only you can know what will make you be better able to not over think things.
I tend to make a decisive decision not to dwell on something (especially something I cannot alter) and especially if it is something that I find my mind wandering back to!
If you feel you do need to give this 'issue' of your place in things some head space decide to do this on a limited basis, e.g. when making dinner in the evening, or just after lunch, etc, a set time to 'indulge' these thoughts or look up stuff on line, or perhaps only one evening a week - or maybe after a group or before it.
I might make a decisive decision to think about something else at all other times; or better still do something else (make a cuppa, send an email, read a book, clean something, do some work) to get that thought out of my head.
You might find purposefully clearing your head of these thoughts works and when it comes to the time to start thinking it all through again, the time you have given yourself 'permission' to mull it over, that at your designated time you realize it is not necessary!
You naturally fit into the mums group, just as I do with dd, who is my birth daughter, and ds who is my son by adoption. But if we were all talking birth stories, sore nipples and C-sections I would have nothing to add about the arrival of my son into this world! This may make me feel left out, so it is you who need to make sure you feel comfortable with the level of sharing and if not, to remove yourself from the group, perhaps temporarily.
At the very start I may have found this hard, hearing about births and pregnancy. But now, three years into parenting him (almost) it doesn't bother me a jot that he didn't take up residence in my uterus for 9 months.
So I would ask your dp to see if you can join the mum's group, make it maybe a trial and if you don't like it, you can leave. In the meantime
I'd encourage you and your dp to make the general group more active. Is it an online group? Is so, post some articles about parenting, birth etc from different perspectives and try and engage others.
Articles about which baby carrier or sling to use, prams, cots, safety for baby, etc, are important to both parents so as long as it is not all pictures of sore nipples etc you may find some of the others start engaging.
Good luck. 