I'm so nervous.

1. I could forget everything.
2. I might forget everything.
3. I can forget everything.
4. I may forget everything.
-----------------------
Can we use all the statements? What is the difference among them? Which ones are commonly used? Is #1 the same as #2? Is #3 the same as #4?

Explanation 1: 'Could' has less possibility and if a person use 'could,' others feel polite.
(Is this explanation right?)

1 and 2 are most commonly used.

3 implies that the forgetting is possible and very likely!

4 is almost like 2, but "might" is more commonly used in everyday speech.