Help, I've got 4 weeks to do a 100 fly

I did a 50 fly in my first swim meet in 20+ years with minimal training so I thought I would have no problem doing a 100 three months later. Since then I have trained harder and improved significantly in the freestyle with speed and endurance yet I still die after 50 yards of butterfly. 95% of my training is free as the fly takes everything out of me. At the end of a 2000 yard workout I might do one 50. That might not sound like much to you all but I can do a set of like 15 x 100 which is well beyond what i have been capable in years. Once a week I might do 10X25 fly or I might just see how far I can go. I always die after the 2nd turn. Complete breakdown. Yet I could do a 50 months ago without any concern at all of just completing the distance. My stroke may be fundamentally flawed. Maybe I just got by as a youth on raw power. The only form I have is all out until the momentum is lost and then it is complete breakdown withing 3 strokes. I don't know if there is some kind of hesitation stroke I can use to get through, but at this point I think I need sets and strategies to build up to it on the stroke I have.