Break Up

My girlfriend and I have been dating for the last 2 and a half years until we broke up last Saturday. Two weeks prior to the break up, we had a really big argument about her sexuality and her wanting to experiment with it. We talked about that over a date and we moved on. Those two weeks flew with no issues and we went out to the beach with each other. That Saturday was nothing out of the ordinary, we were supposed to meet up so I could help her with her work but we didn't know the library was closed until she got there. When she told me, I offered to go to another library if not my place; she declined and said I should take her back home because that day it was hot and she was tired. I got a little frustrated but she said she'd make it up to me the next day and I calmed down, nothing really came of it. I was randomly invited to a party afterwards and I tried calling her to ask if she wanted to go with me but the calls went straight to voicemail. After about an hour or so she texted me saying we needed a break, I asked what was wrong and she called me on the phone crying saying we needed to break up because she wasn't ready for the commitment. It shocked me because we've had no issues and really good transparency between us. A few hours later, she texted me saying she ran out of her lithium and that could've been a role to play there, but said we'd talk about it whenever she gets more of it. Ever since then we've hung out a lot as friends and we've been flirty here and there, but transitioning from girlfriend of 2 and a half years to my best friend is hard to do. We've been labeled as a situationship for the time being, but I'm scared for when we finally do talk about it because it'd feel like I'm being broken up with all over again and I'm not prepared for the life after her. What do I do?