I said good morning to my friend and they replied with:
今言おうとしてたよ
I understood this as,
I was just about to say the same thing
However, I asked them if they could explain the sentence just in case. They said 言おう is not like 言いましょう so I've become confused. What does it mean in this sentence?
You did understand it just fine.
今言おうとしてたよ
I was just about to say the same thing
-(y)ou to suru is "to be about to" and is appended to the stem of the verb. The form 言いましょう is -(y)ou applied to the auxiliary masu.
So, 言いましょう is the polite form of 言おう, but it can't be used to form something like 言いましょうとする.
言いましょう・言おう and 言おうとする are used completely differently. This is probably why you were told they are different.
言おうとしてた is the past tense of 言おうとする。
言おうとする is the volitional form of 言う. Meaning "I want to say that"
言いましょう means "let's say this!" and is directed by the speaker to the listener.
「言おうとしてた」and 「言いましょう」are very different in meaning!
Sorry for adding this as a new answer. (I cannot {yet} add comments) @Earthliη♦
Wouldn't it just become 言いましょうとしていたよ in past polite?
