How can I stand up for myself?

I have never been able to stand up for myself. At all. I hate confrontation and I just shut down completely. I can’t think and I get sick to my stomach. Hell I threw up in the middle of one once.

I’m having problems with a guy at my work. I(F, because he doesn’t see women as equals except females in positions above him) had been working there for 5 years before he was hired. He technically has a position higher than mine. My work is an awesome place and everyone treats each other as equals and acknowledges that every position in our smallish family owned company is essential to our success. Except this guy when it comes to me. He doesn’t do it in an obvious way and no one is around when he does it. It’s just little comments and things he says.
I’m a sales assistant/receptionist. I help with some of the sales orders for the sales people when they are really busy but I don’t actually sit down with the customers to discuss things plus I do receptionist duties like greeting/helping customers coming in, answering phones, taking payments, stuff like that. He is a sales person. It’s part of my job to help with sales so when he asks me to do parts of his jobs I have to help. The problem is he doesn’t actually need help. He does about half the amount the other ones do but the main reason is that he spends a large amount of the day socializing with other coworkers. Well over 20 minutes everyday. He asks me to “help” finish one of his jobs because he gets his rocks off being able to boss me around. He always asks me to do something on days when he gets knocked down a peg for doing something wrong or he messed up on a job. He gets all excited and happy coming up to me asking if I have it done yet. It’s insulting that I’m doing his work so he can goof off and feel like he is more important than me. There’s other examples but that’s the gist.

A situation has come up that I need to confront the situation in the next day day or two and I don’t know how I’m going to do it. Like I said, I shut down. I can’t think of any of my points I’m trying to get across. I freeze up. My mind goes blank. How can I do this?

If anyone has a good way to basically say that I don’t appreciate being made to do his work for him so he can socialize and there has been more times than not that he has spent the entire time I’ve worked on his stuff that’s he’s talked the entire time it took me to do it.

Any help would be amazing. I don’t feel like I’m going to be able to do this and will have to put up with this for the foreseeable future.