How does this one compare?

I have feelings for my girlfriends friend. We all had a child at approximately the same time.
And surprise, we are all having relationship problems..
I know this sounds, and is, selfish and horrible.
Her friend and I keep "accidently" bumping into each other at the local gym, there is a definate spark between us and it us becoming unbearable
She keeps dropping little hints to my girlfriend. To top it all, she is getting married in a couple of months time to a man who she clearly isn't happy with, she has told my girlfriend this.
Wrongly or rightly, I can't stop thinking about her and I know, by the way she acts around me, the feeling is mutual.
What the Fudge should I do???????????????