It is brutal. But you realise it isn't really a "regression" - this suggests a phase that you go through and come out the other side back to how things were.
The newborn phase has a very different sleep physiology. It's almost like a fouth trimester. Sleep is similar to sleep in the womb - in that it's a passive state.
During the first 3-4 months of life, as long as all the baby's needs are met then the passive state is asleep. So baby is only likely to wake when hungry, uncomfortable (dirty nappy, too hot/cold) or in pain. As long as baby has a fully tummy and feels comfy and secure - then baby will enter its passive state of sleeping.
From around 3-4 months old this all changes. It's not a phase. It's not a regression. It's a permanent change.
The act of going to sleep now becomes active. It's no longer passive, parent and baby now have to actively work to get baby to sleep. You also have to work to keep baby asleep because sleep now starts to develop into cycles, so there will be brief wakeful times between one sleep cycle and the next - baby and parent have to actively work to help baby stay asleep through one cycle into the next.
You only come through this stage when you learn a new way of actively helping baby get to sleep. So thats how long the "regression" will last.
Some things parents use to actively encourage sleep are parent-dependant and some allow for independant settling. Some methods are easier to wean off, others less so.
Some ideas:
Dummy
Feeding to sleep
Cuddling to sleep
Rocking, bouncing
Car drive
Pushchair walk
Sling walk/juggle
Swaddle/tightly tucked in
Cosleeping