I was in a relationship for five years, which ended around six months ago. I haven't spoken to my ex since then but keep holding on to this deep resentment towards her. She hasn't reached out, and I know we will never get back together, but I don't know if I have really moved on because I keep thinking about how angry I am with her. What does this mean? Will I ever be able to put it behind me? Am I being childish?
You're not being childish at all. To move on from a relationship is never easy, irrespective of how long it has lasted. You can't expect yourself to forget about the years you spent together that quickly. It may take months or may happen next week. All you need to focus on is what you both consciously decided to end, and what you are looking forward to in the future. The anger is natural, as is the resentment, but it doesn't add any value to your life. It's hard to look at these things objectively, but you have to trust the fact that time and space will help you move on whenever you're ready. Until then, allow yourself this period of grief even if it seems counterproductive.

My fiance wants us to move into his brother's house until we find a place to rent after marriage. the problem is I can't get along with his brother at all. He is rude, tries to control everything, and makes it seem as if he is doing me a huge favour just by speaking to me. Should I tell my fiance how I feel, given that this may ruin our relationship?
Relationships aren't ruined by honesty; they are strengthened by them. Your fiance may not have any idea about your feelings for his brother, because he will always look at him differently. You can always pretend to get along, of course, but that won't last forever. It makes more sense to be honest and look for a compromise.