Dear Anna:
You read like a very reasonable, rational person in regard to the upcoming ending of the monogamous, love relationship with your boyfriend, very accepting of a transition to a friendship, like it all makes sense to you. On the other hand, it reads to me, that your heart is not where your rational is. Emotionally you are not okay with the ending of this relationship.
To not “end a very joyful chapter of our lives on a bad note” you may want to end it sooner than planned, as in the present, before he moves away.
You may otherwise not try to “subdue those negative emotions’, or to subdue any emotions, and instead, listen to them: what are they telling you?
If they are telling you that you are not ready or willing to end the monogamous relationship and transition to friendship, then listen and accept your feelings. Then make the appropriate decisions.
anita