you need to understand how that 6 month METV work
@Kor***
,
The visa is valid for 6 months from the date they issued it to you
It is good for unlimited entries into thailand with each entry getting you stamped in for 60 days.
Once the 60 day stamp is running out you either go to the immigration office and get a 30 day extension for 1900baht
OR
You go to a border, stamp out of thailand, stamp in&out of another country then stamp right back into thailand to get another 60 day stamp.
You do that for the validity of the visa itself.
There's no way you can stay inside the country longer than 90 days (60 day entry and 30 day extension) per entry on that visa.
If you're in Chiang Main, catch the "green line" bus to the Mae Sai - Tachiliek border with Burma. It cost 10USD or 500baht to stamp in-n-out of Burma then back into thailand and you'd get a 60 day stamp.