Should I say I "post a post" on a forum?

It's theoretically correct, you just need to add an article to make a noun out of the word post, like this:

I wanted to post a post, but I forgot to do so.

EDIT:

Either I didn't understand the question, or the others didn't understand my answer, so I'd like to rephrase myself: The above sentence is grammatical, there's nothing wrong with it from the technical point of view.

To go outside of what the OP asks, I would also add that repeating the same word is a bit unusual, but not incorrect. We don't know how the OP needs/wants to use the sentence, therefore we may not judge what's best for him.

To suggest some more usual options:

I wanted to submit a new post.
I wanted to create a new thread.
I wanted to add my reply.

No. (!) Like any other word, immediate repetition is bad practice. "I wanted to leave a post", or "I wanted to post an answer" would be fine. "I wanted to post" is not wrong, but could be confusing.

"Post a post" would be correct, however, you might be better choosing a different noun, because it does sound a little odd. You might say "post an article", or "post a blog entry", or "start a thread", or "reply on a thread", or simply "I wanted to post to the forum, but I forgot to." Your second suggestion of "I wanted to post but I forgot too" is also OK, but it does feel a little incomplete to me.