So I've been playing a DnD 5e campaign for the past couple of weeks, and I've come to the conclusion today that I'm not really having much fun with it. I've come into the campaign midway-through and don't really have any good opportunities to insert my character into things, and I have trouble even getting into the character that I've made outside of using X spell to burn Y enemy, which just doesn't appeal to me. I've given it a couple of weeks, but I find that I mostly am just stuck behind this screen (we use Maptools) for 3 hours each Sunday, watching text scroll by without much care for what any of it says.

So with all that in mind, I'd like to leave the campaign. I don't have any problem with the system, or timing schedules, and while I might not be having as much fun with my character as I'd like, I feel like that's not going to change anytime soon - a lot of the current characters are already established, and I feel like mine doesn't fill any niche that hasn't already been filled. On top of which, the quirks my character does have don't seem to be that interesting to the other players, and I don't have much fun trying to play them out.

In short, I don't really like playing this campaign, but I have no excuse to leave the campaign except that I'm not having fun - and rather than keep my DM's hopes up that I'll be staying, I'd like to leave as gracefully as possible, without insulting his DMing skills.

He's not bad - the group is just kind of large already, and I feel like I don't get much out of it myself. How can I let the DM know this, without insulting his DMing skills?