Breeze through the lyrics. Writing a break up rhythm and blues song should be a cinch. As long as the emotion comes from deep within and contains all the pain of your soul, it should be fine. The blues part of the R&B name comes from the lyrics.
Use 4 beat bars. One of the backbones of R&B music is the 4 beat measures. Usually the last 2 beats are eighth notes.
Develop a backbeat. In order to get the typical rhythm and blues song you need to have a backbeat. Simply speaking, it is just accenting the second beat and fourth beat in the measure.
Throw in some syncopation. Accent weaker notes and vary the points of stress. Use some of the techniques made popular in jazz.
Listen to gospel songs for ideas. Many R&B songs have used gospel and substituted the name of the lover instead of Jesus or God.
Be aware that today much of the music that is Urban African-American music is referred to as R&B. Most genres, like soul, may have their own classification but they still will be loosely placed in the R&B genre.