Please help me!

I am writing my experience in the past to my friend. So I am using past tense.
But if I want to express my thoughts at that time, can I use present tense?
For example,

I was very excited at that time.
How can I speak in English?
How can I .......?
How ...........................?
After thoughts on various trends, I became very frighten.

If you are writing about something that happened in the past. You can talk about it as if it was in the present. You can relive a moment in your past.

There are various novels and films which explore this topic. How to make an event which had happened in the past, come alive?

From what point of view is the author writing?

I am thinking of a film that I recently saw: Raymond Chandler's "The Big Sleep" (1939)

Chandler's ironic tone and extraordinary metaphors focused readers on individual scenes, which he excelled at writing. Many of these evoke Southern California in the late 1930s so vividly that the setting seems to become part of the plot.

The book is by Raymond Chandler, and Philip Marlowe is the detective.

The Big Sleep (1946 film), a film starring Humphrey Bogart