Bryony I have followed your thread with interest. I was in the same position a while back.
Posted here too, asking for advice (different name at the time). I was very negative back then, angry, tearful and jealous. I was really grieving the fact that the family life I had envisaged was ruined (so I thought!). The day the DNA test came back was a real low point in my life. We both cried for a long time. Consensus was that I should probably leave DP, and fast forward 2 years, I still sometimes doubt the decision I made to stay. Like you, I was mid thirties and couldn't imagine life with anyone else and couldn't face starting again.
I thought the whole issue would get easier with time. The truth is it hasn't really, but I am able to deal with it mentally in a more manageable way.
What happened to us is that OW and baby moved abroad, making it difficult for contact to continue. DP hasn't seen his child for nearly a year now. They occasionally Facebook or Skype to keep in touch, but tbh there is no bond, they are more like strangers to whom we pay a lot of money every month. Hard not to resent at times. I ended up leaving Facebook, got so wound up by it all, watching their lifestyle. You adapt and make changes to make it work for you and your life.
Some posters above have suggested your DP needs to make moves before August.. I think that everyone's circumstances are so different, it's not possible to make these judgements.
I admire what you have said in your posts, you seem to have real strength and clarity. I hope it works out for you.
For what it's worth, DP and I are now expecting are own, much wanted little one. Unlike what some have said above, I don't think it's a quick fix to the fact that your partner has had a baby with someone else before you. That pain never goes away, it just becomes more manageable. Sitting in the scan knowing he'd been there before (albeit against his wish) with OW was tough. Likewise, I have similar feelings about the upcoming delivery.
When I'm having a tough day with it all, I just try to blank OW and all the issues I have with her out of my mind, and just focus on the step-daughter and her well-being. Remind myself that she's actually really cute and happy in the pics and appears to be well looked after and being raised in a happy family environment that her mother has created for her. We aren't really involved, but are helping financially and if SD is part of our lives one day we will welcome her. Not a day goes by when I don't think about her, one way or another. Sometimes with a smile, sometimes with tears, sometimes a big sigh.
Being a stepmother puts you on a spectrum with the extremes being either an amazing, selfless woman co-parenting a blended family under one roof to on the other hand being an angry, bitter woman totally losing the plot Jeremy Kyle style. I haven't achieved the former but try not to be the latter.