This is not a pleasant situation in which to find yourself. To resolve the problem, we need to do some bedroom detective work in order to find out what has gotten your boyfriend (fiance) down.
When exploring why this problem has erupted in your relationship, you will need to study both individual and relationship factors. First, check out the individual factors that may be stalling his sexuality. Is he turned on when he's by himself. Or is his sex drive gone all together.
If his drive is gone, period, then he may be depressed or overly stressed. If his drive is fine when he's alone, then we have to assume that the sexual problem is'relational.'
Since the sex was good at one time, then you need to figure out when sex with you went on the fritz. Was it after the engagement?
Your boyfriend needs to work with you to figure out why he thinks that his sexual desire for you has dwindled. There are many possibilities.
Many relationships (including sexual relationships) take a nose dive after a couple has been married or living together for about two years. This is because the two year mark seems to be the length of time required for the 'transference' to kick in.
What this means is that after about two years, people begin to unconsciously see their mates as their parents. When this happens, all the unresolved feelings toward our parents enter the relationship and all hell breaks loose. Anger abounds and sex goes on the fritz.
I keep wondering if your fiance is blocked sexually with you because he now sees you as a controlling parent. Notice that he freaks out whenever you attempt to take control of sex and initiate it.
The fact that he can't handle this indicates that he has unresolved issues in this area. He must have felt overpowered, controlled or wiped out by one or both of his parents. This would explain why he freaks when you approach him.
What's more, by staying away from you sexually, his unconscious mind may actually think that it is preserving his identity (sex creates a sense of blending and merging of the selves). If he hasn't fully developed a strong sense of self, then sex could be seen as threatening, especially if his unconscious has come to view you as the dangerous, overpowering parent who could wash his entire being completely overboard.
I have thrown out lots of angles to follow. Basically, you need to get him talking and exploring about what he thinks the lack of sexual interest means, and how it relates to his history and unresolved issues, etc..
If he refuses to talk with you, then go to see a therapist together. If he refuses to do this, and you choose to stick with the relationship, then you will need to examine what purpose the sexual deprivation serves for you and how this relates to your history.