I had this problem when I started. My instructor told me to aim for the tip of the mouthpiece with the bottom of the tip of my tongue--that forced me to bring my tongue up, stopped the slapping, and got me hitting tip to tip. Practice tonguing eighth notes with, say, high B, as part of your warmup, move on to sixteenths, and you'll soon be touching tip to tip.
You can solve your other problem by practicing very slowly. It's mostly a matter of coordination, not speed. Start out as slowly as necessary, then gradually speed up. You'll improve pretty fast.