Don't panic about diagnosis. yet. First of all, you could take your adult kids to ten psychiatrists and get ten different diagnosis. Psychiatry is not an exact science. A lot of it is opinion. Another thing to think about. If your adult child uses drugs, one can not diagnose somebody until the person is clean and get a true idea of who he or she is.
It is possible the diagnosis is right and they are borderline. But I've been in the mental health system (as a patient) since age 23 and have had so many diagnosis. that it makes my head spin. The two I'm sure are true are mood disorder not otherwise specified and anxiety disorder...and probably a non verbal learning problem. This is over forty years.
Deal with the behaviors in my opinion, not trying to fix them. You can't. I had to fix myself and so do they. I had no loving parent or family to help me when i was in my 20's so I had to do it alone and in a way I really think it was best. My family did not understand and had a lack of compassion (although they all had their own issues, which may be why) and having to stand on my own was frustrating, but it made me more determined; stronger. I do not feel weak now. I am proud of how far I've come to overcome my challenges. Now I am the one everyone comes to when they need advice/a shoulder to cry on. They sense I can understand and help with sound suggestions, perhaps...
Like drug addiction, your adult child will do better with his/her mental illness once he/she is committed to working hard, going to therapy, taking the right medications (if needed), staying clean and not giving the psychiatrist/psychologist grief. Work hard and you do the best. Admit you need help, chin held high, and work at it. Motivation must be there.This is first hand knowledge. If the person will not accept the diagnosis and not try very hard to change, he or she will not change.
Nobody caused these issues and you are not going to change your grown child. It is up to him or her.
Hugs for all the hurting hearts out there.