How do I stop fumbling my words on the radio under pressure?

Under pressure the words tangle, the pace runs away from you, and the phraseology you know cold in the quiet deserts you when it counts. That is not a lack of ability. It is what stress does to a mind that has not yet made the words automatic. The skill is there. Pressure is just getting to it first. The fix is not to try harder in the moment. It is to over-learn the phrases in calm, until they come out right without you steering them. When the standard call is burned in deep, pressure has nothing to disrupt, because you are not composing, you are reciting. Drill your core phraseology out loud, slowly and correctly, when nothing is on the line, a few minutes a day. Speed comes after accuracy, never before it. Build the groove deep enough in calm and it will hold when the pressure comes, because your mouth will know the way even when your nerves do not.