Most Helpful Guy
You need to calm down. Freaking out about pleasing a guy that badly is going to turn off guys way faster than sending a thank you text after a date. A thank you text is appreciated, and seen as a kind gesture. It doesn't come across as desperate. Excessively worrying about whether or not every little thing you do is going to be 'right' for a guy DOES come across as desperate though, and that's what will sabotage the relationship. Subverting your own personality to try and make it perfect for the guy is a disastrous plan. It never works, because eventually you will need to start acting like yourself again, and you will be exposed for being emotionally dishonest all that time.
You are trying to find a partnership. That means you both have to be into it, you both have to have your emotional needs met. Subverting yourself to impress him is not a partnership proposition, and you will be building a foundation on sand. Calm down, relax, don't worry so much. If he likes you, he will like you. Just don't spaz out and you'll be fine. He won't lose interest because you were polite. Unless he is a huge, throbbing asshole.1