Please, please don't end it in an email. I got dumped via email three months ago by my secondary partner after two and a half years with him. It's cowardly as hell, insensitive, and cruel. Just meet her in a public place (so she can't make too big a scene), gently tell her the romantic/sexual relationship is not working for the two of you, and offer your continued friendship. Also, hope to hell she isn't knocked up and that she hasn't given your partner an STD. You will want to
use condoms with him now until you have his next STD test results. Remember that some STDs have an incubation period. If she is barebacking with your guy, she's probably doing it with others too, so make sure he gets checked out thoroughly.
I personally think dating on your own is a much better way to find someone who is truly right for you. The odds of one person being compatible with both of you are extremely low. Anyway, if you aren't sure you're bi, why not just date men? Why force yourself into sexual/romantic behavior that doesn't really ring your bell?