In my expert opinion you are indeed having ruined orgasms! Which is what the clenching and the loss of horniness is about. You’ve just got to stop earlier my dear. However, before you try that, I’d suggest you explore a bit and really play with seeing just where you can stop, it does vary. Don’t worry about if you go over, and if you do, have an orgasm, but before that tease yourself and have fun seeing how far you can go and not get the clenching.
What’s likely happening is you have a bit of delayed response to being on the edge and you’re tipping over into a ruin (a really weak one). So your other task is to experience HARD ruins. You do that by really building up hard to cumming, often a combination of clitoral and g-spot stimulation is the best way, force yourself to hold back as long as you can and then push yourself over into what would be an explosive orgasm and THEN wrench your hands away and ruin it. It’s awful, and fucked up and wonderful.
Let us know how you get on!