I'm seeing a lot of this from her side. You pushing to try FWB when she was clear about wanting sex in a relationship was really cruel given that you know she's got feelings for you. She's not "girlfriend material" but apparently good enough to use for sex. She should have told you to go F yourself, but because she's got a thing for you, she hoped that you may see her in a different light.
And now for whatever reason, she still loves you....and you think she can be a "friend" while flaunting your new AMAZING girl in front of her. Hate to break it to you, but she's your 'friend' only because she's settling for the crumbs you give her. Her feelings to you won't turn off just because you say you just want to be friends.
The kind thing to do would be to NOT be her friend. End things with her properly and give her space to get over you. Yes, she should have made this decision to distance you herself....but she hasn't, so you need to do it. End the friendship and cease contact on social media. Yes, she will think badly of you but there's nothing you can do about that except acknowledge the fact that you also made some bad decisions here.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.