An explanation tells people why. An excuse asks them to stop asking.

The words can be identical and the purpose different. An explanation hands over information and leaves the other person free to respond to it; an excuse is aimed at closing the subject. You can usually tell which one you are giving by whether you want a reply or an ending, and the listener feels that difference immediately.

Before you explain yourself next time, ask what you want to happen after you stop speaking. If the honest answer is that you want it over, say that instead. Asking for a subject to be dropped is reasonable and more honest than dressing it as a reason.