I think it's important to remember the reason why you left your ex-girlfriend. Things weren't working out between you and although you were in love with her and dated for 2 years, it stopped working. Now that you're happily with someone else, leaving her for your ex is risky but things might not work out a second time either and if that happens you'll be left with nothing. You've made your decision to split with your ex and date someone new so stick with it and lead your life looking forward, not back.