Options to overcome phentermine addiction?

Addictionologist. You should see a board certified addictionologist for help. I have treated many with amphetamine dependency. There are some medications which can help with the lethargy and depression until that goes away. I also recommend narcotics anonymous. Your success with treatment will be directly related to how much effort you put into it. It's hard, but you can do it. There's still hope if you're trying.
Lower & shorter. Use a lower dose (1/2 tablet) if effective. Also limit use to three months. More importantly, view your transition to a healthy eating pattern with smaller portions, fruits and veggies, etc.