Relationship advice, different desires in life?

My boyfriend and I have been together for 8mo. He's 26, I'm 20. Marriage has been mentioned, and honestly I don't know. I don't know if I want marriage, and I don't know if I'd be good for it. Also he wants kids in the future, he said far future… But I don't want kids at all. I know people chance over time, but I will never change my mind about that. Kids are an N to the O. I feel like if we get married then we will just get divorced over the kids issue. I love him, but I don't know if we are going the same direction in life. He's also more on the clingy side, which is causing me to lose feelings every now and then. We talk it over, get our spark re-kindled, and keep going. But it's happened more than once… How much longer is it going to go on? The rest of my life if we get married? I don't want to break up with him, I'm too involved emotionally to do that. I just can't. Any advice?

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