Maybe if you realize and reflect how lucky you are to have such parents, you will find it easier to say thank you to them. Our relationship with our parents is like our relationship with Hashem. Hashem iis also our parent, and we are also supposed to thank him for everything that He gives us.
Parents love their children unconditionally,and will always give to them (Just like Hashem loves and always gives to us.) It is a good middah to learn how to thank them. I was learning over shabbos in a sefer, Tiv Haemunah by Rav Gamliel Rabinovitch. He talks about how everything you receive is from Hashem. So why do we thank people for things if they were just the stick in Hashems hands? Chazal say that one is who ungrateful for the goodness he receives from a friend is tantamount to being ungrateful for the goodness bestowed upon from Hashem. Why though? Wasnt the person just a glove of HKB”H? Because we live in a physical world, we see observe and comprehend things on a physical level. So when we see with our own eyes that someone did something for us, we need to show our hakaras hatov. But we must also raelize that he was a messenger, so we need to thank Hashem as well. They are an emissary of HKBH.
If you start thanking them, once, twice, three times, it will get easier and even help you on a higher level as well 🙂