I know it's hard. I used to really, really struggle at the beginning of my poly relationship too. I used to question whether or not I could handle it... but, with trial and error, some soul searching, and a lot of communicating and work... I finally feel that I'm on the right track. Time, experience and knowledge will help you figure out if being poly is something you want/need, or if it just does not fit right with you.
Have a read through this amazing article that Gala Girl first posted - it really helped me to put jealousy and insecurity in its place: