Dating your ex's best friend

Is it a bad idea to date your ex's best friend? He's my best friend also and I have developed feeling for him emotionally and sexually and I can't ignore it. But my ex is his best friend and so I know it's wrong But my best friend has always been there for me when my ex broke my heart multiple times. And I kinda fell for him. Basicly I'm crazy and don't know what I'm doing with my life. HELP