First-timer dealing with issues

I’m looking to get locked up for the first time, and my keyholder (assuming that’s what they’re called) is helping me along. I picked one up from Amazon—yes, really—and went with the largest size available. Getting it on has been a bit of a two-step process; I have to lean forward so my testicles can pop out properly. But if I try to straighten up or sit back in that position, it starts hurting. I’m keen to give this a go for them, but I’m puzzled by the discomfort. Isn’t that supposed to be fine?