if I miscarried under the same circumstances but I was DSD's mum instead of just being DP's gf, DP would've had no problem leaving her at her granny's to be with me. Why was I left to deal with it alone just because I was the gf and not the mother of his eldest child?
I'm not really sure you are comparing apples to apples here OP.
If you were the mum instead of just the girlfriend, all sorts of circumstances would have been different.
- You would have had a higher level of commitment to each other (cf. not living together at the time of the pregnancy)
- You would presumably have a solid family unit of 3
- You would have all 3 likely been at home together, not DP and DSD staying with his mum
- The miscarriage would have started and progressed while you were in each other's company
- dare I say it but this would not have been your first pregnancy
Maybe there are other deeper reasons to his decision-making process on that night that he hasn't yet shared with you, that might explain and help you understand.
- did his mum know about you then?
- did she know you were even pregnant?
- did / does she like you?
- how would she have reacted to his upping and leaving in the middle of the night?
- how would he have explained it, to her and to DD?
- did DD know about you / how well did you know her?
- had DD ever stayed over night at Grannie's by herself?
- was DP already "invested" in the pregnancy or still getting his head around it?
- had he ever had experience of another miscarriage?
There are probably some mitigating factors somewhere, you just have to find them.
He is not a bad man, therefore he wouldn't have made a jerky decision on purpose. At the time he thought, on balance, he was doing the right thing (or at least the least-bad thing) for everyone involved, including his PFB. So if he can help you understand his thought processes, maybe you can resolve your resentment.