I have read somewhere that 'have' : is a state verb, so we don't add '-ing' to it. For Eg: 'I am having a car' seems okay to non-natives like me . However, I know It is incorrect to say that. My question is,if that is so why do we say 'having said that.....' or I am having to work hard.
While "have" in the sense of ownership rarely appears in the progressive aspect, it can certainly take the "-ing" ending as a gerund.
Having a car is a big responsibility.
But your examples aren't even the same meaning of "have".
Having said that...
is an example of a perfect participle, where "have" is taking its role as an auxilliary verb. Meanwhile
I am having to work hard
is another sense of "have", which is the "have to" construction that expresses obligation. This is probably related to the ownership sense (the same construction appears in Spanish using the analogous verb), but in practice it's a separate thing. And this isn't even getting into other phrasal usages of "have", like "have lunch".
So, yeah, "have" has a lot of different uses in English, and many of them can take the progressive aspect.