Use what works for you, and trust that. Don't get caught up in doing the therapy "perfectly". The way to do the therapy correctly is just to do it, daily, and to apply what you are learning daily, as much as you can. Rinse, repeat. Use the expressions from the therapy that feel right for you at your particular time. Right now it might be the STOP statement. Later you might find yourself changing how you say that or becoming less resistent about it, in a way that you might say "Oh, it's you again..." I think in the beginning maybe we do need to be a bit more forceful about it, but don't confuse this with fighting or getting angry. I'd say it's being firm, and alerting your brain that you're making a change. As you start to become more automatic in stopping ANTs, then maybe later your statement for new/other ANTs will reflect where you are then and how you feel.
Trust yourself. You're doing fine. And by asking this question, it tells me you're doing the therapy. Good for you!