When was this, exactly?
In the second half of of 13th c. the Livonian order wasn't able to seriously defeat the Lithuanian or the Vladimir princes even once. The high watermark was occupying the republic of Pskov, followed by decades of constant beatdowns at the hands of Alexander, Dovmont, Mindaugas, Treniota, Yaroslav and Vaiselga. If either the Russians or Lithuanians could afford to keep an army in the field for longer than a few months or managed to secure some siege engines, we wouldn't even be talking about the Livonian order today as anything major.
Which incidentally brings me to my one suggestion: to keep Belarus you need to build two or three fortresses the size of Riga, and have the Tatars go all Batu v.2 on Russia to distract it in the meanwhile.