~なくする negative verb and する grammar

Question

一人ぽっちでいられなくしたのはお前 (Spoken)

Is this just like the なくなる grammar (e.g. 食べなくなる) but with する implying it was done by someone?

Answer 1

～くなる and ～くする are fundamentally different. For example:

• 赤くなる
  to become red / to turn red
• 赤くする
  to make (something) red

Likewise:

• 一人ぽっちでいられなくなる
  to become unable to be alone
• 一人ぽっちでいられなくする
  to make [someone] unable to be alone

And the original sentence is a cleft sentence where お前 has been pulled out for emphasis.

• 一人ぽっちでいられなくしたのはお前(だ)。
  It's you who made [someone] unable to be alone.
  ^{(i.e., It's you who made [someone] prone to suffer from loneliness.)}

Who this "someone" is depends on the context. It can be a third person, the listener or the speaker themself. If this "someone" is the speaker, the sentence roughly means something like "You made me unable to live alone without you anymore."