Complicated

Me and my roommate have been best friends since highschool we where dorm mates. Both went to CU boulder. Both are from Oregon. Madeline and I'm Aspen. We have been going to the gym together working out. Treadmills a yoga class Tai chi and a cardio class we go twice a week.
Madeline is beautiful five foot four. Pale raven hair pale skin built like a barbie doll. I'm five foot six dark red hair shy. My mom uncle's Aunts older strangers Everyone tells me I look like a young wynonna Judd. I had to look it up but I kinda do.
I've always been shy around men I'm just nervous I guess I don't know.
But madeline started dating a guy from the gym.
They call him rockwall but his name is Justin. Tall and really muscular hints his nick name. Dark black former powerlifter. Sweet neat smart cool guy.
I enjoy his company he hangs out sometime waiting for madeline to get home from work or school. We both work we both go to school for our graduates.
My confession is I've been sightly prejudice most of my life. I felt guilty that I saw justin as my equal in intelligence.
Now I feel guilty because for the past three months I've been getting dicked down power fucked daily almost by justin. He is not as big as I thought he would be the whole bbc rumors but almost eight or nine inches. I enjoy him fucking me and when he asked for a threesome madeline choose me it was fun yet nerve wrecking I swear she could tell I fuck him all the time.
Justin buys us both lingerie and sex toys. He has a huge sex drive it's impressive. But I need to tell her because I'm developing feeling for him. I know she is into poly relationships. But I've been sneaking and being a secret slut. Help what or how do I approach this