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I think being honest is the best approach. One game I'm playing in has been going on for three or so years and the dm is getting bored of it. Though the players all still love it including me. He told us he's going to stop after finishing off the story in a reasonable manner and by then we will have another game lined up. I would recommend you do the same thing. Tell them all you want to stop, but tell them you're going to stop after resolving the story in a reasonable way. It helps if you can tell them how many sessions you think it will take to finish the story/campaign. Then try to get someone else to dm a new campaign afterwards or maybe you can start a fresh one if no one else wants to dm.
Personally I know from experience that if the dm is not having fun for multiple sessions then the quality of the dming will go down quick. Well at least that's how it was for me.