What is the difference between these two
I have been thinking about you.
I am thinking about you.
"I have been thinking about you" is past tense, and implies that you're not thinking about the person any more.
"I am thinking about you" is present tense, and indicates you are currently thinking about them. If used in a letter, this version indicates a person remains in your thoughts.
'I have been thinking about you' says that it has happened, and may or may not be still happening, whereas 'I am thinking about you' is present tense, it's happening now, and may or may not have happened previously. The latter can also be construed that it's happening on a regular basis, dependant on the context of the rest of what's said.