Try to forget about the guitar whilst playing the mandolin, stick to its natural tuning, persevere with the reduced finger board space, treat the plectrum with a delicate touch, learn the scales(especially D,G A and F)and if the instrument is an oysterback, get comfortable.
If you give yourself time and apply yourself to learning before trying to short cut and give less than competent renditions you will reap the rewards of good schooling.