Who does Romeo forget about when he beholds Juliet at the party?

Rosaline his old muse/obsession.

Romeo sees Juliet at the Capulet feast and falls instantly in love with her. Rosaline is simply a long-lost memory at this point. Even Romeo admits that he never really saw what true beauty or love was until this night, that he beholds Juliet. It is at this point that Romeo himself realizes the difference in the love he thought he felt for Rosaline and the love he now feels for Juliet.