I learned the struggle of learning two languages that are very similar. Im currently learning Danish and Norwegian full time now (not the right term, but you guys understand) and I always mix dreng with gutt when " "speaking" danish and gutt with dreng when speaking norwegian. Any advice on how to solve this, other than trying harder?
Awhile back, I had similar difficulties. I decided to concentrate on one language at a time (for now), and decided to revisit other languages at a later time. I selected Danish for the difficulty, thinking that if I can learn it well, it may make learning Norwegian and Swedish easier in the future. Best wishes. :)