Rhythm strumming and fills

It seems every other dulcimer player I hear, even other beginners, seem to catch on quickly to rhythm strumming, and adding rhythm strumming and fill ins to their melodies. I have been playing for 3 and a half years, have several books and have watched seceral youtube videos, and these skills still completely elude me.

I have built a decent repertoire of melodies that I can play well, and some of them sound fine as just melodies, but some of them just sound dull and very lacking. I would love to have a fuller sound.

I would love to have a personal instructor, but 2 things stand in my way. 1. I just really can't afford one. 2. There aren't any dulcimer teachers that I know of within a manageable driving distance of where I live.

Can anyone offer any help or advice?