What do I do?

okay, so let me start off by saying, I'm bisexual. my mom knows and she doesn't care.
anyways-
I met this girl about a year ago, and we dated and stuff. lets call her shay. and it was a mutual break-up, because my mom wouldn't let me see her, because I...

got into trouble with skipping school and so on. but I recently started to see her with my friends and stuff. we've been hooking up for the past few months, and I'm head over heels for her again. and she is for me too. but then she met my best friend. lets call her lola. my best friend knows the history I've had with shay, and she asked me if it's okay to hookup with her and stuff. I told her its fine, but it will hurt. they are now crazy about eachother and I don't know what to do. I mean, im sitting there, watching them hold hands. they both know their hurting me, but what can I do. im HEAD OVER HEELS for shay! but I also dont want to hurt my best friend. please give me advice on what to do. I've came to a conclusion, that when lola breaks shay's heart, shay will come back to me, and I'm going to be here.
lola wouldnt even have a relationship with a girl, and shay wants a relationship!
she's a slut with guys, she's had s*x with over 18, and she's only 16.
she's not even bisexual. she's doing it to fit in with my friends, and me.
I dont want shay to get hurt, but I want her to learn her lesson.
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please helppp

Ok ok let me be the first to say this,

You shouldn't have given your friend the go ahead to date this girl you really liked. By doing that well you kinda stepped aside, and you can't blame your friend or this other chick for liking one another, im not saying you are but it sounds that way.
The best thing I can tell you is, move on. Yes that may be hard or even impossible but if shay moved you so can you don't be hateful ok seriously.. life is too short. <3

The other answer posted was great.
Your friends may not know what theyre doing to you.
You told them itd be okay,so maybe theyre unaware of your feelings?
I'd try talking to them about how you feel.Make sure they both know your feelings towards them.And if Lola is just doing this to fit in,sooner or later Karma will get her,Shay will realize it,and things'll most likely work out for the better

I think you should sit down and talk with your friend (lola) because she might not know that she is hurting you - sometimes things that seem so obvious to you, may not seem so obvious to others. Don't bottle all of this inside, you need to talk to them both about the situation. Your kind of beating around the bush with this situation and you really need to just tackle it and take it on.