I've been playing the bass guitar and the trombone for almost 10 years, and writing music (but not lyrics!) for about half as long. When I wanted to get a certain musical line correct, I'd sit there and keep playing it again and again. I still try to do that, but I guess I have less patience these days.
Performing: Tough when everyone's watching you. Only done it a few times, and not sure whether I'd want to keep doing it, but maybe.
Playing in a group: Difficult at first, but fun. Especially in bass lines, I can relax and support other players, while they do their thing. Played long blues jams in a friend's basement in high school. Probably more fun than actually performing it.
In private: I really like it. I try to put my mood into my music. I make up something simple, or I play something I know that fits my mood. Improvisation is not easy for me, and when I've done it in front of an audience, I've always wanted to stop when it doesn't sound the way I meant it, and try it again. In private, I can do that as much as I want, so I do.
Maybe the hardest part is expressing emotion through the music. I invest emotion when I play, and I feel it, but I doubt others can feel or hear it.