Every extra sentence of explanation sounds a little more like an apology. Say it once and stop.

Over explaining signals doubt even when you have none. The first sentence states your position, and everything after it reads as bargaining, as if you are hoping they will agree so you can stop talking. People hear the length before they hear the content. A short answer sounds settled because it does not need anything from the listener.

Practice ending sentences earlier this week. I cannot make it, is enough, and you do not have to supply three reasons. When the urge comes to add one more line, let the pause sit there instead, and notice that nothing bad happens.