Keeping the Rhythm
The trick is to play what you play in the proper rhythm, even if you only play a portion of the song. In other words, practice keeping the rhythm going at the expense of everything else. Don't play the cords, or play one or two notes in the cord rather than the whole thing. Before long, you will be able to get more and more notes included in the cords, or you will learn which notes are optional. In most cases nobody but you will realize that there is a note missing.