I'm finding this quite tricky spikeyone. I'm not sure what help 'they' could have given you. Your perspective isn't consistent. If you were vulnerable, had no money and were totally dependent on your mum I'm not sure how you think you could have looked after the baby. Yet you said that you were capable so I don't understand why you didn't make plans to move out before the second attempt and actual abortion. You said it was clear that your mum wasn't going to let you bring up the baby in her house.
'They' should have called 'someone'. Who should they have called? Did you tell the medical professionals who to call maybe the father of the child to take you away so that you could be away from your mother? Where was the father in all this? Was there someone else they could have called? It sounds like you were very dependent on your mum and she wouldn't support you. What about your dad?
Would you have wanted social services to take you and house you somewhere so you could raise your child alone? You said you had no money so that would be hard.
If you were capable of looking after a baby alone why didn't you do something yourself? After you were sent away after the first attempt you went back to your mother's house. If was clear she wasn't going to support you with the baby so why not move in with your boyfriend's family? Move out alone?
From the medical perspective they refused to operate the first time and sent you away. Then you, an adult (you say that you were vulnerable yet capable of making decisions) came back a second time and signed on the dotted line albeit in tears. I'm sure plenty of women having a termination cry for the loss of a baby yet know its the best choice.
From your mother's perspective I can imagine seriously trying to dissuade my DD from keeping a baby if the father was abusive or awful generally (you don't say if this is the case). Otherwise we could all be stuck with that person in our lives pretty much till the end of our days. Much better to terminate, get rid of abusive man, then have DC with someone you want to spend your life with.
If this is still haunting you nearly 20 years later you need proper counselling to help you to come to terms with it. You can't turn back time and change anything so you need to be able to move on and live the rest of your life. Don't let the past ruin the future.