Self-bondage Challenge
There actually is one way to reduce the total pain you have coming, but you are not going to like it.
If you are brave enough, you could move abruptly away, so that many clothespins come off at the same time. The intensity of the pain will be much greater this way, but your brain will be too overwhelmed by the pain to feel each individual clothespin come off. Try it if you like, but many people who attempt this find that they cannot do it. Your sub-conscious isn't stupid, it knows how much this will hurt, and it will resist sending such orders to your muscles. So jump away quickly for those few who are both willing and able. For the rest of you, I am really going to enjoy watching those clothespins come off, one by one by one... Let me just sit back and enjoy the show now. I do so love my job!
You can take breaks as you need to, but you cannot quit. So just rest up for a bit then get back at it. Don't press "Continue" until you have pulled yourself far enough away to remove all of the clothespins, and you have retrieved the key and released your handcuffs.