As growlycat suggested, getting help without your parents knowing is probably the easiest/safest way.

If that doesn't work, is there any way you can say something like that you are feeling a lot better, but there's one or two left over things (maybe mention something, like you get anxious before exams, so it affects your performance in school a bit or that you are having trouble making some decisions in life, there's many reasons why somebody needs a therapist, make up one that doesn't worry them or surprise them). Since the therapist is not allowed legally to tell your parents (as far as I know for the US), that might be a good option.

Also, forgive me for bringing this up, but 6 months is a very short period of time to get better. Especially if you mention you have a history of abuse, and have been using coping mechanisms like self harm for a long time. Just be aware that getting to a good state of mind in 6 months will be very intense and hard work!