She's not with another guy, she thinks of you all the time. Give her space, the gift of missing you

Man up and be the fucking player, think of it as chess or poker, don't let your emotions cloud your judgement. She's probably thinking of you way more than you think, just don't let your insecurities get to you. Don't get needy and clingy with the messages, just control the urge to contact her.
Never forget you two hook up because both of you want it, not just because you want it. Give yourself some credit, understand why she's attracted to you and give her more of that. Please, please, don't make the mistake of losing yourself over a girl, it's never worth it, never ever.
In the end, f you're miserable all of the time since you're hooking up with her, something is wrong with that picture. There are succubas out there, emotional vampires, just be careful.