Well, first of all a game designer is not necessarily "a guy with great ideas". He might be, but he must, above all, be able to structure ideas and build the non-existing game given certain specifics. Second, do you find yourself losing interest because no progress is being made? Is it because you're going at it as a hobby? If it was your job to sit in front of a computer and work out game mechanics, the challenge of it all, "dance" with all your given restrictions, etc - would you be able to do it (do you see yourself doing it)?
Now 3D art, that's completely different. You basically sculpt concepts given to you, having into account poly counts, loops, and again - restrictions.
Level Design is, to me, an art. It's knowing how to make the level playable and direct the player where he/she should go, challenging him/her along the way and in the end the player feels like he/she's improved. Depending on the size of the studio you work in a game designer might end up doubling as a level designer.