I appreciate your dilemma. I'd say your teacher could be helping you with ear training, which is appropriate much earlier in your learning, instead of sticking so long with strictly rote memorization.
I recommend you spend some time with a basic step: Finding melodies by ear on the banjo neck, note by note, one finger on each hand (no roll or even chord at first). Once you have the melody notes, you can add the chords, and then use a forward roll (or the Foggy Mt. Breakdown roll) to see if you can combine melody with a roll. Sort of like either: "including in the roll the string with the melody on it," or else: "Play the melody, and where there's filler space, put in some rolling notes."
This process is trial and error. Lots of chances to be confused. Be sure to leave out melody notes that are giving you trouble, and also keep the most accurate rhythm you can, avoiding putting too many or few notes in the spaces. On my web site, there's an article about making a play-along practice tape. That is a very useful way of rhythmically guiding your arrangement as you build it.
Another thing to be aware of: It's fine and quite normal to have the arrangement come out differently at different times. Lots of ways can be considered correct. Eyewitness quote from Earl Scruggs: "I have no idea how someone could play something exactly the same way twice in a row." Something to think about. (Something besides memorization is at work.)
At my Basic Skills Banjo Camp every January, I teach this skill to as many of the campers are ready to take it on. I'm sure you would be one of the folks to figure it out pretty quickly, and then move on to your second arrangement. After that it gets a lot easier, especially when you learn to incorporate licks and phrases lifted from the many arrangements you've already learned.
Please read the article on my web site called Teaching Beginners. It emphasizes the need to start playing with other relatively new people as soon as possible, in slow jams. It also shows how to find and even organize slow jams that have teachers for supervision. This will give you a chance to learn accompanying skills (not just soloing). I’d recommend my DVD, Bluegrass Jamming for that as well.
Best of luck with your picking. I think if you just jump in, this part of the process won't truly stymie you, just frustrate you for a while! Comfort yourself with the notion that EVERY good banjo player has to go through this step. It's kind of when your right hand learns to "think" Scruggs style, instead of just "reciting" from memory.