I only think you would be regarded as an apostate if you rejected the Triune God. (for example if you became an atheist, a Muslim, a Jehovah's Witness, or something along those lines)
As far as a heretic...yeah, maybe. Canon 751 defines Heresy as: "...the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith;"
The tough word in there is "obstinate"
The New Commentary on Canon Law says that you must have "male fide" (bad faith) and you must do so with "full knowledge" to be really guilty of heresy. In other words, you have to believe that it is true and reject it anyway.
That is not to say that you don't hold heretical beliefs. But for you to be guilty latae sententiae of the crime of heresy, you would basically need to be intentionally leading somebody (to include yourself) astray, knowing that you are leading somebody astray from the truth. I, frankly, don't believe that most fallen-away Catholics are in that position (some are...but those are by far and away the minority).
Having said that, I think it makes some ex-Catholics feel really good to believe that they have personally been anathematized by the Church and that they were thrown out on their ear and condemned to everlasting flames of doom...and if it makes you feel good, please, by all means, carry on.
The New Commentary on Canon Law says that you must have "male fide" (bad faith) and you must do so with "full knowledge" to be really guilty of heresy. In other words, you have to believe that it is true and reject it anyway.
Isn't that a definition of crazy? To believe something is true AND reject it?
I reject the Catholic only doctrines because I believe them to be false.