Without asking, you are already living the path of the "no". If you realize that, it becomes a lot easier to change your mindset.
I'd even say it's not about being hardened by rejection, but rather standing up for yourself. Speak up. Dare to ask something. Dare to want something. Then learning that taking risks increase your chances of getting a "yes". The famous "you lose 100% of the shots you don't make" :)