I have a wide vocabulary but....?

I have a really hard time articulating my words when I'm in direct conversation with someone, unless I speak very slowly. I thought at first that it was a result of my teeth being crooked so I couldn't pronounce things as quickly as I needed to, but I noticed that it goes beyond that-- that as soon as I begin thinking* about speaking all other thoughts escape me, and I fill my sentences with "like" and "umm" so much that people stop listening, and whatever argument/point/story I have becomes uninteresting to others, because it takes me so long to say it. Sometimes I can think about what I want to say, before I say it, but then the moment is gone. I don't think I have any disorder, and i don't have a stutter, but it is a problem, especially because I love talking with other people, and I can never sound respectable. I think sometimes I might get too excited, but others people can talk when they're excited... What sort of exercises/experiments could I do to try and make thing easier?