I know it is difficult to accept that a family member has a mental illness and then accept that they may not have the ability to fulfill YOUR dreams for them and their future. It is difficult to let these dreams go, but they are YOUR dreams and not HIS dreams. It may be that you are inadvertently shaming your son. He probably know that you are disappointed in him. This can do great damage to someone with a mental illness. They WANT to be normal, but the truth is, they are not. They need to cope with life in a way that is counterintuitive and "common sense" for people without mental illness. Have you ever heard of NAMI? It is the National Alliance on Mental Illness. Try to get in contact with them, or even just go to the website. They can give you a lot of great information and guidance on how to "help" your son. Their are local chapters as well.
In many ways, you are doing a lot of things right. Try not to be "embarrassed" by your son. He cannot help that he has a mental illness and it only adds to the stigma of MI and to his own shame. Try to accept him as he is and love him as your son.
I am not saying that his using is acceptable. It is far from it. But, it is how he has learned to cope. He needs to learn new skills. Try not to judge him for what you perceive as immature decisions. It is likely just part of his illness and you will need to accept that, however, allow him to make these immature decisions so that he can experience the consequences of them is very important for his recovery.