Please clarify me this sentence.

Hi EnglishForward guys,

Can I ask you for help?

Let's suppose this conversation between the girl and boy.

Boy: I wanna invite you to the restaurant.

Girl: No. I don't think I'd better.

Please does it mean here "I don't think I'd better" something like....I think I shouldn't do it?

Or I found this example:

“No, I don’t think I’d better. I will listen to it or read the book itself or whatever eventually but I don’t think I’d better do it today. I am sick from this, do you understand? I am physically ill. You don’t understand the impact this has had on me.”

I think it really means "I shouldn't do something", but what is the difference between but I don’t think I’d better do it today / but I think shouldn't do it today.

Can you please clarify me this phrase? Or could you give me other examples?

Thank you in advance.

With Regards

JCD