Do you have any advice for someone who lost their pain tolerance in a vanilla-heavy sexual relationship?
First of all, I want you to abandon the idea of being insecure about your pain tolerance (see also: Comparing). BDSM is about a lot more than who can take the most hits. It’s a relationship, not an olympic sport.
That said, I’ve struggled with this myself. All kinds of factors can influence what you can tolerate on a given day on a given part of your body. Hormones are huge too if you’re not on hormonal birth control. When I’m ovulating, you could basically run me over with a car and my panties would be soaked. But when I’m PMSing or on my period, a light spanking will make me cry. Pay attention to your body. I first noticed this about myself when I noticed I hate using my dry brush when I’m PMSing. See how your body reacts to the same factors applied on different days.
If you want to gradually up your pain tolerance, a useful approach is counting out switches. For impact tools I’m trying to get better at taking, my Dom will tell me “you’re getting 10 switches with this” and have me count them out. Some kind of magic happens in knowing the number is finite and the more you play with it and gradually increase the intensity, the more you’ll get used to it. The human body does very well with gradual increase. You don’t go into the gym after a winter of sitting on the couch and run at top speed on the treadmill for an hour and expect not to burn out. You add on slow. This is the same. Start a journey of adding on.
And if your journey isn’t with a consistent partner, front load this information before a play session when discussing your limits. Say, "I want to work on my pain tolerance, so this is what I need” and spell it out. Make sure the person knows you’re trying to expand your limits and, if you do reach a limit, knows and honors your safe words.
And if you’re working on upping your pain tolerance solo - may I recommend a vigorous dry brushing?