Hello @zoulja,
this is unfortunate. I hope you can explain in more detail what is hindering you, so we can make it better.
About rolling back, if you have made a backup of your mongodb as mentioned in the upgrade nodes you can just use 3.1.x with that backed up database.
Otherwise there is no rollback process implemented in graylog by now. But if you tell me what problems you have with the 3.2 version I will try to make your live easier as soon as possible.