Why am I getting tired of talking quickly? I am giving a lecture and I feel how difficult it is for me to speak. This is my favorite topic, but it takes me 3 minutes.

Dig in two directions:

1. Learn to speak a lot and continuously, as a result, learn to breathe correctly and articulate correctly.
2. Train your brain so that it gets used to the fact that you have to talk a lot and does not clog you with fatigue. I mean that giving a lecture should be psychologically comfortable for you.

I am giving a lecture and I find it difficult to speak.

This is a topic for discussion with a neurologist. The sooner, the better. Similar sensations occur when there is insufficient blood supply to the brain area. This is how ischemic stroke often manifests itself.