An infant and/or child not loved unconditionally and skillfully can develop an attachment disorder. Such disorders can be worked on in therapy and progress can be made. And i believe that you are correct: those who develop attachment disorders as children, if untreated, do tend to raise children with attachment disorders. It is not certain that this will happen but the probability goes up.

With regard to letting them go: i think your t is speaking of when the person wants time alone or to develop skills or other friends or to end the relationship, and you allow it to happen, you don't resist, even though it might hurt.