Oh shittt what the hell were you thinking man. You set yourself up for this double-edged knife situation.
Joanna has the right to be angry she broke up with her BF Tom because YOU said you had feelings for her. It's your fault dude. You have to realize that first before you can deal with her effectively, going all "How can I get rid of this crazy fan girl" attitude will just make her even more pissed off.

You have to apologize to her sincerely, just explain that you got jealous when you said you had feelings for her. That's the only way out.
Try to empathize with her, how would you feel if some girl made you break up with your GF just because she wanted your attention ?

Otherwise if she finds out about Stef, she will definitely tell on you, and all her friends how you tried to woo another girl while dating Stef,
believe me buddy "Hell has no wrath like a woman scorned" I'd advise you to quell down the situation before the shit hits the fan.