If people were only as smart as us...
Anyways I know what this is like, and it is really hard to control. First you get mad as you can't believe that you were just told this. Then you can't answer as you feel like you might start yelling if you open your mouth. Then you tell yourself to calm down as stress is bad for the disease.
I think there are a number of ways to deal with this thing. Having the support of family (fiance) is definately something you should realize you are lucky to have. Someone who understands is better than a lot of people have, so take advantage of this when it is needed. Just let it out when you need to... let him know why it upsets you and what you would like to tell the others who say the ignorant things. Just telling him what you would like to have said may help you get the anger out.
I think we need to realize that most of our family have really no idea what we go through. It is not there fault, but they have other things to worry about. I am not trying to marginalize our disease or your condition or their worries, but we all have the trait of focusing on our own issues. Family usually does not think out exactly what they are saying or understand what effect it is having on you. I suggest doing one of a few things. You can always turn the "attack" back on them, by doing a bit of an attack also. You can say something along the lines of "Oh! I did not realize that you have been dealing with Crohn's all this time also and know what I am going through!... (continue to make the point using sarcasm)" This way is not perhaps the best way to handle the situation, but it may be a wake-up call for people who keep bothering you about it.
What I see as a more productive way of dealing with this situation is telling the family member in question EXACTLY what you feel. Tell them why what they are saying hurts you or is wrong, and why the thing you miss most is being able to have a normal job, etc. Tell them the exact reasons why this disease makes it hard for you to do whatever they are telling you to do, so that they understand that this is a real medical condition and you are going through real symptoms. "I am tired" does not sound as convincing as "I am tired due to the anemia that I have developed due to intestinal bleeding". The more detail the better. Even if your family says something like "Thats not what I meant" or "I did not ask that... that is too much information" let them know that the continued questioning is only answerable this way so that if they don't want to know the answer, stop asking the question.
If this continues to go on I recommend reminding them of the previous conversation where you explained to them your thoughts ("remember two weeks ago I told you...") or even let them know that you already previously told them the answer to your question, and if they want further information they can contact your Gastroenterologist (even if you just see a doctor I think it sounds better to say GI). Chances are your family would be too embarrased to do this and you are then in the clear.
Lastly, you can also try and limit the questioning by asking for favours after each question
. This may sound funny, but next time you are asked "Why don't you get a job" consider telling them how your disease affects you and end by saying "But I could really use some time to recover. Would you be willing to do X (watch the kids, pick up groceries, etc.) for me next week? Chances are the questions will soon stop if you constantly ask for favours after each one
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Good luck with this and just remember that no one realizes what they are saying, and they are not trying to hurt or upset you. While we can't control what others think, say or do, we can control our own emotions.