Whilst I have never tried a Flex Bar, I have had a reasonably serious case of TE. My tricep and forearm (inner and outer) got to the point where I could not use a screw driver or pickup a cup of coffee. I am a coffee loving service technician so you can see my trouble. As I have played for nearly 20 plus years I am reasonably sure it is not technique related.
I have found that even with exercise and not playing regularly it tends to flare up. Point being, you should always go easy when undertaking rehab until you are certain that the injury is becoming bullet proof. Otherwise, you can go around in circles.
Never give up.Never,never,never,never.