"Wanting" or "want"? (Stative verbs: participial clauses; present continuous usages?)

Lately I have noticed that a lot of people use "wanting" in sentences, or in books, but I don't get it because my English teachers have always said to me that with verbs like "love", "like", "want" etc. we can't write the verb ending "-ing". But how it is possible that it's in book then?

Some examples:

• She reached her hand out, wanting to touch him...

• Not wanting to talk about it, Clary turned...

• Actually, I’ve been wanting to ask you how...

I really want to know where I can use it and where I can't. It really drives me crazy that I don't know it.

These sentences are fine, because the -ing form is used as an adjective:

"She reached her hand out, wanting to touch him..."

"Not wanting to talk about it, Clary turned..."

What your English teachers probably meant was that ordinarily we do not use these stative verbs in progressive constructions, like this:

"I am wanting to ask you how ... "
"I am liking this job very much."

But sometimes the 'state' which these verbs designate is conceived as subject to change over time; and when that is the case a progressive construction becomes acceptable. In your last example, for instance, the state is about to come to an end:

"I have been wanting to ask you how ... "

Or in this case, the state has been increasing over time:

I am liking this job more and more every day.

ADD:
Jez objects that ‘native speakers might well say "I am liking this job very much."’ Perhaps so—the progressive construction has been steadily increasing its scope for 300 years now, and it is possible that the punctiliar sense on Facebook has definitively ‘unstatived’ like. But I would not expect to hear this except in a dynamic context as “I’m liking this job now”.

So I would advise Learners not to use the progressive construction. The stative sense is still built in to the word itself, and you can’t sound wrong if you use the simple present; but in some contexts you may sound wrong with the progressive.

My first experience of hearing the "I am wanting to..." form was as a native of NE USA moving to the SE. I saw it as a colloquialism at the time, something like "I am fixing to...", but it does seem to be becoming more common elsewhere, too.

I see the "rule" of stative verbs as a general rule saying that in most cases such stative verbs are not used in progressive form. But all the same there may be situations where a progressive form can make sense as in your example 3:

-I've been wanting (all the time) to ask you...

There is a second thing to consider. In English people are so accustomed to using progressives forms that sometimes progressive forms are used of stative verbs for emphasis or whatever. The rule about stative verbs is derived from logic - with some verbs there is no reason to express progress - and from observation of their use. But there is an arbitrary zone where it is difficult to say whether a verb is stative or not. And people don't carry a dictionary around to have a look if the verb they want to use is marked as stative or not. After all a verb has no special feature that let us know that the verb is stative.