OP,
If you've struggled with writing and a therapist suggests that you get tested, then I would.
The Lab School of Washington used to offer classes for adults, but if you look under "Night School" in the directory, there's a contact person listed:
http://www.labschool.org/page/Directory
You may also want to look into ASDEC:
http://asdec.org/services
Since it's writing that seems to be the sticking point, you may not have to go through the whole neuropsych evaluation.
Also, if you have an LD, you're in good company. All of these highly successful people have dyslexia:
http://dyslexia.yale.edu/successfuldyslexics.html