Okay, so you confronted your long-term, problematic poor performer. You put the fear of God in him, and he went for some sort of help. What ever it was, it worked. Now he wants a different relationship with you, but you're still not over your anger about his irresponsibility for the past several years. You need to get over this attitude and fast.
This is not your spouse who suddenly decided to get sober. I have seen relapses happen over this sort of thing. The problem: You have a mindset and a well-honed "neurologic" pathway in your brain of seeing your employee as a loser. Now, you need the help. Not him.
This is the same syndrome that is experienced by family members when an alcohol or drug addict finally recovers--seriously recovers. Do you realize that you can, by your attitude, create a situation where your employee relapses into poor performance again. If this happens, you will of course blame the employee, but what you did was PROVOKE IT.
So, it is normal and natural to get mad at your employee for wasting years of time and along with it lost productivity. You won however. You salvaged your employee.
Often, supervisors feel that a troubled employee put the workplace through a long period of disruption before he or she finally sought help. Consider the following in order to acquire a more positive attitude and lessen the likelihood that you could act provocatively and jeopardize your employee’s progress: 1) Assume employees want to do a good job and feel good about their work. 2) Believe that some personal problems do not easily permit self-diagnosis or self-motivated decisions to seek help. Instead, some personal problems and diseases are naturally fought with denial until consequences nearly overwhelm the victim or others. 3) With this understanding, focus on the present and reinforce the changes made by your employee. This will increase productivity and help you develop a more supportive attitude.