I've had one of these! DD didn't sleep before 11 until she was about 5 or 6 months and then we gradually moved her back - 10.45 for a few days and then 10.30 for a week.... and eventually she was settling at 8ish, but I think not until more like 8 months (when her day sleeps shifted significantly). By 2, she was pretty reliably going to sleep at 7.

DS1 did much the same but then all of a sudden he was tired at 7 and when put down, stayed down and I think this was earlier (maybe 4 months) too.

And DS2 was all over the place.

I guess I'm saying that I think it's a variation of normal and that I am absolutely certain that like everything else, it'll pass. I would tend toward not pushing againste it too hard and maybe just bringing it back in increments if you can.