I read this in a hurry, but if I read this correctly I think I can sympathize with where you're coming from. If I understand what you're saying correctly, you're afraid to talk with people sometimes because you're not sure how to say things appropriately, or don't always know what is appropriate? If so, I feel that way often.

There's definitely a difference between saying, "You're such a jerk. No, I won't do this," and "Sorry, but I've got a lot on my plate now, so I'm not going to be able to do this," or even "Sorry, but I'm not comfortable doing this because (reason)." It's socially acceptable to decline requests, as long as you do it in a "nice" way. If you do your part-if you are polite-then if the person takes offense, it's their issue, not yours. Finally, I'd perhaps try to talk a bit with your fiance's parents? I don't think you should see them if it makes you uncomfortable, but at the same time, if it means a lot to him, you've got to take that into consideration.

If that's not what you meant, I'm sorry. Let me know if I misunderstood.