By the sounds of it, on wonder you aren't 100% up for it at the moment!
Your man should be understanding of your feelings and see to himself if you don't feel like it.
Yes, I'm a guy and yes, I should be telling you to stop being selfish etc, yadayadayada.
however, My g/f is currently pregnant and our previously healthy sex life is now a once a week occurence. She's tired 24/7, lacks confidence with her appearance and thinks I shouldn't find someone so 'fat & hideous' sexy. (Her words, not mine).
When we do manage to 'do it' I make sure she is relaxed, a warm bath, massage and that usually does the trick.
If you want to have sex with your man, try to relax first. It should never be pressured or rushed as you will never enjoy it.
Also, think about the things that turn you on and get your man to do them for you.
If your guy still gives you stress over not wanting sex then kick his a*se into touch as he obviously has no thoughts for anything other than his libido.