If I act really psychotic, he'll come back to me.

Evening affirmation. Repeat in your own head ten times at 8pm and at least twelve times at 10pm. Don't move your lips, dammit! After each session, call your ex and scream at him or her. Then, drive by the house of the person you suspect they are seeing, letting your car idle in front of the house until you see movement inside the house. Then speed off! Later, go to a bar and get drunk and cry to strangers about how mistreated you were and how perfect your relationship was. Swear you're going to get a restraining order against your ex and their (possible) new partner. Call your ex and appologize for your behavior and swear you love them and that's why you're so mean to them. Remember to cry a LOT. If you do this often enough they'll come back to you, I promise. And you'll live happily ever after!