I've really struggled with feeling like the outsider as an SM. I still do.
I was incredibly naive looking back! I moved into what was once the family home. The 3 DSDs were older than my DS, they were teenagers. Quickly and perhaps too late I realized that there was no 'equal playing field'.
I was the outsider, when I raised issues I got extreme defensiveness for daring to question the 'status quo', which was their status quo. DP got a lot of flack from his ExW and daughters from any challenges I made within our own home. It reminded me of the psychology of the 'out group' - it makes an 'in group' feel more bonded - so I stumbled into actually making DP, his daughters and ExW feel more like a family simply because I was different and questioned their normal.
Reading these boards, it seems that a lot of happier step families are the ones who
- move into a new house, neutral territory.
- where the Ex has moved on with a new partner.
- where the kids are young enough to be flexible.
- where the Dad isn't being guilt tripped.
- where the SM takes time to allow kids to adjust, but also does asset some authority in a fair way to all.
- where step parent and parent forge a new 'team' - rather than splinter apart pulled by children or Exes.
I'm just trying to learn from my experience I suppose!