First, you never have to feel guilty for wanting sex and you never have to have sex if you aren’t feeling it! The problem is, you might worry about your partner feeling rejected or neglected. Working together and communicating your needs is so important in a partnership (I don't have to tell you that, though).
Here are a few things you can say to your partner when you’re not in the mood:
“I’d actually really prefer if we could cuddle a bit, I’d love some intimacy that way right now.”
“You make me feel so special when you express [that you want me, etc.] but I think I’d prefer if we [did this] instead.”
“I’m really busy right now doing [xyz], how about we check back in later and see how we’re feeling?”
When it comes to how your partner feels...
You can always start what you are saying with something like "I really don't want you to feel rejected" or start with a statement that shows your partner how much you appreciate them making you feel wanted.
Ultimately, you cannot control your partner's reaction, but you can work on a system that allows you both to communicate what you need in a way that feels good for both of you.