Advise please 😪

I need a way to tell my mom I've been sexually active , I'm afraid she will look at me differently like she did to my sisters. 
This all started for about 3 months now and the guy I'm with I'm happily  with him still , we've been dating since 8th grade (I'm a junior now ) 
I'm 17 .. 
This is a difficult situation because I've seen the look in her eyes when my sisters told her and i don't know what she did or talked to them about but one of them left crying and left to live with her husband now . 
I need words , letters anything that I can own up to it .. I've never felt this scared and guilty beforeÂ