I have been in a similar situation CiderRosie and it is very hard. In addition, being treated in this way by someone who professes to love you will also likely be damaging to your self esteem and confidence.
My (ex) dp wanted to keep his new life (me) completely separate from his old life (ex-w and dd). Just like your dp, he said it was because ex-w was angry, unreasonable, difficult etc and confronting her would only cause a huge ruck which would accomplish nothing other than increasing the strain between them and badly affecting his dd. So, every weekend and every Wednesday, he would go off to be with his dd, and I would be left to occupy myself.
For many months (years even), I admired and fully supported his commitment, but eventually it became ludicrous. His ex-w blamed me for the marriage breakup (they were living apart but not divorced when we got together) because in her logic, our relationship meant that he would not return to the marriage as he had 3x previously. Her argument in forbidding me to spend time with their dd was that dd couldn't risk becoming attached to someone who might not be a permanent fixture in her father's life. He didn't argue.
Another poster said that it appears your DP wants it all on his own terms, and I agree with that completely. As long as I was a patient, tolerant, supportive, understanding, loving 'good girl', my dp stayed. It finally dawned on me that this wasn't a transitional/adjustment phase but how my dp wanted things to be permanently. My 'turn', our turn, was never going to come.
I agree with other posters who support your DP/his ex sitting together at school/family events, because they are his dd's family. But, if this far along, your dp has failed to integrate you fully into his life, it shows a shocking lack of commitment and respect. I'm afraid you have to question whether or not you are willing to live like this.