I was fourteen when I first got pregnant, So I told my Mom I missed my period and she was shocked.
I am so sorry that you are struggling with another possible unplanned pregnancy. The abortion you had at 14 obviously had a deep emotional effect on you, even though it was a decision that was pushed onto you by your Mum and your age.
The first thing you need to do is find out if you are definitely pregnant. Then it would be good to talk to a pregnancy advisor to go through your options and think carefully about this very important decision. You may feel at nearly 17 that you can make your decision more independently, and I think it's important not to be pressurised by your boyfriend, or by fear of your Mum's reaction. You definitely need some support and help.
If you ring the national helpline number we will try and get you in touch with some support, and the advisor you speak to will be able to talk to you on the phone about how you are feeling.
You could also look on the Care confidential web site and see if there is a centre near to where you live.