You can let him go, you just don't want to let him go RIGHT NOW. You said you have headaches because of him? that are mild symptom for loving wrong person. Sooner or later, it will become heartache or broken heart. You can let him hurt you (by stay with him) till your heart cannot endure it anymore, you will get tired and able to let him go (but it will cost you a lot of pain). Or you can start to AVOID HIM AT ALL COSTS right now and MOVE ON.
Bay_Cole22