havin feelins for a woman aint simpin, its part of life, its part of bein a man. Even a stone cold pimp puts a little love and soul in his step. Simpin is when you handle your feelings like a bitch, or play yourself by being unjustifiably over-emotional for lack of rational self control, simpin is thirst, simpin is putting the pussy on a pedestal. That doesn't sound like it applies to your situation, and time will fix your issue, so just enjoy life while time takes its course, enjoy other women, and give as little fucks about that female from your past in the mean time.
Also, one thing that always works, socializing with a dime, and on the low at a party or some shit you'll probably both be at, bringing her around the ex and keeping it cool like you have no interest in the ol' girl. Ol' girl will catch feelings and you'll feel like a boss.