Self-soothing is an over-used term in infant sleep. It was never intended to mean what it's come to mean. The term was introduced in the 70s, when researchers concluded that infants rarely slept 5+ hours without waking; instead, some(!) infants would briefly wake & return to sleep without crying. As the first to write about it, the researchers named the behaviour “self-soothing” (vs “signalled” awakenings).
Once in the literature “self-soothing” became many parents’ goal. Sleep trainers jumped on the bandwagon & started to convince parents this was something that could be taught. For a fee, they sell the idea that babies must be put in their cot “drowsy but awake” in order to fall asleep unassisted. But, when a baby is put down in this way, they either wake up straight away & start crying, or (temperament dependant) they are already calm & simply fall asleep.
Babies & young children CANNOT calm themselves from a state of intense stress or heightened arousal. In fact, the brain regions responsible for self-regulation isn’t FULLY formed until around 25 years of age. What researchers actually meant when coining the term, was that some babies (some of the time) can fall back asleep without intervention. This is likely because their needs are met, they are easy-going babies & they are already calm & content. Babies who exhibit this kind of sleeping behaviour from birth are RARE.
Eventually all children, via natural developmental progression, learn to self-soothe, but it cannot be taught. Some babies (& parents) respond well to the strict routines & guidelines imposed by sleep training programs. If it works for you, great! Keep going.
However, it is not a good fit for ALL babies. To suggest otherwise is outrageous & wrong. If the impossible goal of ‘teaching’ your baby to ‘self-soothe’ is doing your head in, please know that you’re not doing ANYTHING wrong.
There's huge variation when it comes to all manners of infant behaviour, least of all sleep. These differences need to be respected & nurtured in whatever ways work for YOU & YOUR baby.
References: Anders & Weinstein 1972