anon32, you are in a very difficult position, and while the advice and stories here are worth listening to, only you can make a decision with regard to having a child. Saying that, you are with your dp for 8years and any decision should really be made by both of you together. Does he really know that you do not want children? Does he understand why? Maybe if he can try to understand why you don't want children, and if you can understand why he really does, then you might come to some solution.
Here's my bit!!: I never wanted children. I had a lot of other things going on in my life. I loved my life and what I did. I was never maternal in any way, and had no interest in children at all. dh always wanted children, lots of them. It was always a bone of contention between us, but tbh I just kept putting dh off, saying 'maybe' but really saying 'no' in my mind (dishonest, I know).
After being married 6-7years, I came around to the idea of 'maybe'. I don't know what happened to 'change' my mind, I just wasn't as opposed to the idea. I never felt like my clock was ticking, never felt a longing to be a mother, but we said we'd give it a go.... and ds1 came along

I know its a cliche but life has never been the same! The irony is, dh realised how much work was involved in a baby, and would have quite happily stopped at one, but I couldnt wait to have another... and ds2 came along

More ironically, I would quite happily have had another, but dh really didnt want to, so we made a decision together that 2 was plenty.
I absolutely adore my children, but still have no interest in children (other people's!!) in general if that makes sense!!
Good luck with your decision, talk with your dp.