how do i tell my mom about my 15 year gap relationship how do i tell my mom about my 15 year gap relationship

hi everyone! i’m 20 (F) and my partner is 35(M), we have been seeing each other for almost a year and a half now and secretly pretty much living with each other.
my mom is my best friend and we do everything together, i am home for a few months and need to tell her about this but i just don’t know how because i’m afraid of how she will react and act towards the situation.