The issue is that's its never 'just' a clump of cells. It's got complete human DNA, can only possibly become an adult human being just like us, and is constantly growing via mitosis. Human life begins at conception because no human life can exist without undergoing that same process from conception to birth.
Hopefully in the future we have artificial birthing technology advanced enough to allow us to transfer the embryo from the mother's womb to an artificial womb. It's not fair to the mother that she be asked to endure a pregnancy she didn't want. But it's even more unfair to the new life that we destroy it when it can't weigh in on that conversation. The sad reality of the world is that sometimes suffering can't be avoided, and nine months is substantially less than permanent termination when it comes to how much each future is affected. Obviously if the pregnancy endangers the life of the mother then more drastic measures have to be undertaken.
Finally, one thing I don't think is ever talked about enough with the abortion debate is foster care. Our system is utterly screwed up. Abortion needs to be ended, but with that HAS to come a huge revamp and improvement of foster care. Once a child doesn't need the mother specifically to survive we cannot possibly expect her to care for it if she doesn't want to or can't. It's a two-sided coin and we need to deal with both.