Not an English teacher, but I will take a go at it. The first sentence is definitely incorrect. --- but now I reread and see Andrews answer, I am not so sure. Fear is a transitive verb. It needs to act upon a noun. That means you can't put another infinitive verb (with to) next. If you put a gerund on the end (-ing), then the verb has become a noun and it works. The correction you were given uses the main verb to be (I'm->I am) so that is why it works. I hope that this helps.

Both are technically correct. A is far less common however, because infinitives sound abstract in spoken English, thus why most native speakers would say that B is correct - A is fine though, just not really used.

I don't know that it's incorrect but it just sounds weird, bro. That said I don't see how I could argue that fear should be different from other verbs in the past tense like wanted, needed wished, desired, etc. So I would like that an English teacher could say something about this, also.