Rain, there are many things about trumpet playing that do not come "naturally". Tonguing is no exception. There are tons of exercizes in just about every trumpet method ever written. They all work the same way: teach your body new habits. The biggest problem with conventional trumpet methods is that they are written for students practicing regularly AND getting regular lessons from teachers that know better. Turning these methods into self help is not as easy as many people that have received formal training imagine. My suggestion to you is less takataka without the horn and more dagadaga with the horn. Clumsiness and brute force with the tongue are two very big reasons that our tonguing is sluggish. Try and make the tonguing as light as possible, start slowly, preferably with a metronome and make sure that the tonguing is very even. Once you can play 16ths at about quarter note = 132, add practice without the horn or mouthpiece to augment your playing with the horn. Take a glass of tap water into every practice session, when your tongue starts to feel thick or your throat starts to tense up, take a sip or two to relax! Before your tonguing WITH the horn has become a "habit", practicing WITHOUT the horn is simply confusing your muscle memory and slowing progress down! After you have your 16ths clean at quarter=132, post again for part two!