Dude, you had all this time up to now to try and pursue Stacy to get back with you. No, you started a relationship with Amanda. Now you see Stacy moving on and seem very happy and that pisses you off. Are you jealous perhaps because the relationship with Amanda did not go so well?
You asked what you should do. You should wish her well (but maybe to yourself, because I doubt she wants to actually hear from you).
If you can't handle me at my worst, then you sure as hell don't deserve me at my best. ~ Marilyn Monroe