tense - Confused! Are there any differences between "I have to go", "I had to go", "I've had to go", "I get to go", "I got to go", "I've got to go"?

I am Confused!

Are there any differences between "I have to go", "I had to go", "I've had to go", "I get to go", "I got to go", "I've got to go"?

In which situation is each term used?

I checked this dictionary http://www.ldoceonline.com/dictionary/get, & found that

get to do something: to have the opportunity to do something

Seem like if you have a opportunity to do something, then use "get".

The expression is in present but why we use present perfect tense (Ex: I've got to go) here?