YOU CAN DO IT!!!!
It sounds to me like you have the internal motivation and if you take it day by day, step by step, you can persevere. These message boards are really helpful for some external motivation. Just a suggestion --- if you give your fiancé specific ways he can help (or things you don't want him to do), I think he can be a good support for you. Weight issues are tricky with our significant others. I know my husband is leery about answering questions about whether or not I have lost weight, knowing that he can easily offend me. I'm sure your financé really cares about your happiness, so if this is important to you, I think you can get him on board.
It sounds like a big goal to do the 90-day Beach Body Challenge. Remember to recognize that this is a life-long journey --- not just 90 days. I have to remind myself of that every day. When I get to a weight that I am happy about, or when I think I am "finished," I quickly remember that I am never finished. Fitness and healthy eating have to be part of our every day life.
There is my 2 cents. For what it is worth...... I have confidence in your efforts and I really hope you can accomplish your goals! Last year I lost my "baby weight" and my "baby" was 6 years old! So, good luck!