The student visitor route was discontinued on 24 April 2015 and student visitor visas were "rebranded" into Part 3 of the Immigration Rules. At that point your visa automatically converted into a "short-term student", and those visas are governed by Paragraphs A57A - A57H of the rules...
Rebrand the student visitor, and extended student visitor routes into
new routes for short term study, which sit in Part 3 of the Rules, so
that the routes are conceptually clearer for those undertaking short
The rule pertinent to your question is covered in Paragraph A57D (b)...
intends to leave the UK at the end of the study or at the end of 6
months whichever is sooner.
So you need to figure out which of those two events happen sooner. But... Because you were caught in a transition, you may wish to challenge it. In that case you should arrange a consultation with a solicitor.
Trying to sign up for more courses is unlikely to be successful and may damage future applications. Because you are a visa-national, it could get complex fairly quickly.