Yeah this needs years and may not fix the issue. Some of the most virulent “gay bashers” back in the day were those who were actually gay and couldn’t emotionally process the identity, and turned that inner conflict outwards into violence against gays. You can’t predict how this would be handles and I think there may be better solutions for the issue.
This approach is, as people have noted, similar to an obsession spell, which are famous for backfiring. One thing that happens is it drives the person mad and they become aggressive, and turn that on you. Another is that it affects you not the target, or as well as the target, in this case making your own discomfort worse.
I’m hearing this is the crux of the problem:
You want them to be more understanding instead of hostile? Attacking them tends to make people more defensive and more hostile, even if it’s just subconsciously … I would suggest you try Dantalion to influence their opinion and get them a new perspective. Or we have a pretty good tutorial for influencing thought patterns here. Same caveats that if their sense of self is strong not much will happen, or they could hide it as they work through it and fix it themselves, but any seed of sympathy for your status as a victim of body dysmorphia can help.