I had this question as well. He was doing better after rehab and then Ally's manager gave him that speech. He looked so dejected after that. It hit him hard, Jackson knew the manager was right and it was only a matter of time before he would mess things up again by drinking, Jackson saw suicide as the only way to prevent that from happening.
But then again he seemed depressed through much of the movie, the drinking to cope with depression and the ringing in his head turned him into an alcoholic.
The "ringing in his head" was probably meant to be ringing in the ears, or tinnitus.
A lot of rockers suffer damage to their hearing or degrees of deafness, after years of hugely amplified performances. IMHO the ringing was meant to signify that his hearing was damaged and would only get worse if he kept performing, making him fear for the future of his career.
Not really. If the thought hadn't been there, Rez wouldn't have been able to move him in that direction. He was harsh, but he was just trying to protect his client and himself. Jack could have chosen to ride it out, lay low, clean up his own act and move on.