RE: Too much draw weight/KE???
What you are describing is exactly why I suggest people start with an AMS retriever reel. It sounds like you lost an arrow. Just 4 more arrows lost and you would have paid for the AMS reel and saved youself a bunch of headaches.
If everything is perfect 80# line is OK. I used to use it. But if anything goes wrong or hangs up, it will break like it was kite string. I never even felt it. It broke that fast and easy.
Although Iused a 70# recurve for fishing for years, it is way more draw weight that needed. I know a lot of compound shooters that are in the high twentys to low thirties for draw weight and theytake hundreds of fish.
One thing you said that is curious to me was, " I had left a few feet of slack ". You don't want a huge amount of slack. The line should make a nice "U" shape, but definately shouldn't be hanging to the ground.
I'd do two things if I were you. First, I'd change that line to at least 200# fast flight. Second I'd crank the draw weight down as low as it will go.
Then if you're gonna try a few practce shots, pick a place that is shallow and has a sandy bottom. Shoot 'til you're comfortable with the set-up.
Good Luck, Have Fun and Be Safe.