my best friend is dating my crush :| ( they think I don't know)

Let's start off with the fact that, I never care if you're gonna date my ex, honestly many people have asked if they could date my ex and I seriously don't care, sure take my leftovers kinda thing, it never bothered me. And i'm also like the intimidating and chill type, I look intimidating apparently but I'm the chillest person you'll meet. So my best friend, bro I've been best friends with her for 18 years, like basically my whole life, and we've been through thick and thin, and me and this guy are like some what good friends, and I had a feeling he liked me, so everyone told me I should ask him if he likes me back, so they said it so much I finally did it, and he said no which I'm totally chill about, like I can't make a guy like me, he must have a different type, so it didn't bother me and we stayed friends, and not like it wasn't upsetting it was just a bit, but I can't get mad over that, that would be so freaking weird, and my best friend said that I should move on, she said he wasn't that good anyway, so I listened to her, and about 2 weeks ago, she was acting like sooo weird and that confused me b/c on Saturday we always hang out and head to the gym, but she couldn't and I asked her why she couldn't and she said she was busy, and I'm not the type to be like 'busy with what' like okay, you're busy ig. Then Monday comes along, and we have gym together, and I texted where she was at, she said she was home sick (also we have gym first period) and I was like well that's weird, and I told her to feel better. During the gym period, I went to take a water break and I heard some weird ash, and there's this little hole that you could see through under the bleachers and I took a little peep... I was curious. And I saw my so called best friend kissing my crush. I took a picture and then I felt so sick I went to the nurse and I called my mom to pick me up, and my mom was like yeah ofc, and I cried for a good 4 hours, and I ended up telling my mom, and she thinks I should drop her, but how the heck am I supposed to tell my best friend that I saw her kissing my crush without crying. I never give a heck about what my friends/ acquaintances do with my ex, but dude my crush? That crossed the line, and it hurt me mentally. She was the one to tell me to move on? But she went to kiss him. But I don't wanna lose her. I know that's wrong but I care so much to the point where I wanna leave, but maybe I can forgive her. But it sucks, like tons. It genuinely hurt me. i have no clue what to do.