The pain is necessary.
It's what tells your body to build calluses. Play till you can feel a blister start to build under your skin. Stop for the day.
Play again the next day, not to that point, but let it hurt.
After 4 or 5 days of it, you will see the skin thicken and the pain will start going away from there on.
Fresh calluses will go away fast! 2 days off will erase 5 days of pain. So stay at it even if you just press your fingers on the strings as if you’re playing while you watch TV or whatever.
Don't play till you can see the blisters or they pop, that is an injury that has to heal. But they seem to build faster if you play till you can feel the blister.
It hurts, but in the long run....... it works.