Frustrations of a spanking sub - getting too little or too much

I wasn't spanked as a child. I started getting really hot to be spanked as an outgrowth of my enema fetish. I never found a life partner who was willing to explore spanking with me, so I had to rely on an occasional visit to a professional, and didn't live close enough to any of them to develop an ongoing relation. I was consistently frustrated by getting spankings that either just weren't hard enough, or even if they seemed pretty hard at the time, a few minutes later my ass wasn't stinging any more. As a sub, wanting to be "unwillingly" spanked by the domme beyond my comfort limit, it would have ruined the experience if I had outright told her to spank me harder and longer. So that has been a frustration for me. ON THE OTHER HAND, I'm quite sure there are women out there who would make me sincerely wish I'd never let them spank me. I haven't experienced that yet, but I'm sure the possibility is real... and actually, part of the excitement about being spanked IS knowing that risk exists, that I might be spanked so hard and so long that I might NEVER want to do that again. I dream of having a regular spanking partner where we could work these things out together over time, but that's not going to happen in real life.

I'm not sure if this relates in particular to anyone else's life experience, or... if you have suggestions how to negotiate a suitable level of pain and discomfort on a first (and possibly only) encounter with a spanking domme, especially when you really don't know your own limits. (Maybe ask her to spank you until you're forced to use your safe word? Except I don't really want to be the one to decide when she stops!)

Like any consensual sexual activity in which pretended lack of consent exists, finding a way to communicate your needs without words that break the mood is the key. Sure, you want a safe word that stops everything, but you also want a way to indicate to your spanker that you want more, harder, etc. as well as the opposite.

Are there mood-appropriate phrases you can both remember that would work? Something simple you wouldn't otherwise say would be best.

Would "I'm not sorry" signal that the spanker should not pause or stop? Might "I'm sorry, I'm really sorry" signal the opposite?

I leave it to you to come up with similar phrases for what you need.