Don't worry about your ex. Also, your 15 years old, he's not the only guy out there that you'll fall in love with. Trust me, I know from experience. I'm 19 years old and 16 months pregnant. I was with my first boyfriend at 14 and I stayed with him for over 2 years, I was sure that I'd be with him for the rest of my life, but now I look back and laugh at how stupid I was. Right after him I dated a guy for 9 months and loved him also. When we both went off to University I was devastated and thought I would never find anyone else, once again, I look back and laugh because a month later I was dating the guy that I'm still dating today. He is my baby's father and now he is NOT wanting this baby. This has caused me to move 5 hours away from him because he has caused so much stress on me, which has also caused stress on my baby. I am now realizing that I'm much happier without him, so once again, love has played a game with me. This is about YOU and your BABY, not your ex. Do not let him make any decisions for you.
I know telling your mom will be a scary thing, but you might be surprised at her response to everything. When I told my mom I was sure that she'd just break down and cry, but instead she was excited. Of course she was also shocked, but now my entire family is excited for this baby. I wish you the best of luck and I hope you choose to tell your mom really soon. I'm here if you ever need to talk.