I'm scared to reveal this to my bf..?

my bf and i are both 16 and were truely in love with each other.. we've loved each other for around 9 months bt we just started "officially" dating about 2 months ago. anyway i've been able to tell him so much and open up to him about so many things bt i've decided i need to tell him like the biggest thing in my life i'm just so scared he's going to treat me differently or act differently around me at times bc so many people have done it after they've found out. my mom died of a brain tumor when i was 2 and my dad got remarried to my stepmom who i pretty much consider my mom bc she's all i've ever known it's just the fact that my mom died.. a part of me has always been missing me and i still miss her everyday and will randomly break down and i really want to tell my bf i hate that he doesn't know and i want him too so badly i'm just so scared..