I dont know what to do. Advice?

I have a boyfriend. To my parents dating is a BIG DEAL! My mom was pregnant with me when she was 16 and she thinks if i date ill do the same thing. Im 14 and ill be 15 in february. She always says she trusts me but not the boy. I wish she would just realize that she raised me well enough to know right from wrong. My boyfriend is my best friends step brother. We have been togethet for about 4 months and my parents dont know. If they find out theyll ground me, shun me, be so dissapointed in me, yell at me, and everything would be a dissaster. My dad says that i can date when im 18 and out of the house and cab take care of myself. I cant help who i fall for. He asked me out and i said yes and i love him too much to ever break up with him. Ppl say im too young to feel love but i know what im feeling and i dont care what anyone says. So heres my question. What if me and him date untill im 18 and then i could tell ny parents abd it wouldnt be a problem, right? He says he wants to marry me and if we ever do then my parents are gonna ask how long ive known him. And theyre gonna ask how long weve been dating and what am i supposed to say? We just started dating or weve been dating since i was 14 and u didnt know about it this whole time and i lied to u my whole life? Theyll hate me! I dont know what im supposed to do!