What is the difference in using" I want you to know..." versus "I wanted you to know..."?

Both are correct but, as so often, it's a question of context:

I want you to know... is a direct way of bringing something, often unpleasant, to somebody's attention.

I want you to know that you are likely to fail unless you study harder.

I want you to know that you can no longer depend on me for financial support.

I wanted you to know... often implies that something had arisen which deterred the speaker from informing you earlier. It is also more likely to refer back.

I wanted you to know that the firm was facing bankruptcy but I hesitated to upset you.

I wanted you to know that you were adopted but I felt that you weren't prepared.

Both are correct. I'd say you want to use the first in this context. "I want you to know" sounds like you're talking to them directly, and "I wanted you to know" puts you at a bit of a remove, like you're explaining your state of mind when you were writing the letter. It depends on how forward and direct you want to be I guess.