How do you express the phrase "was going to" as in "I forget what I was going to say"?
A usual interpretation is собирался (to do something ).But we also use хотел in this case meaning intention.
I forget what I was going to say--Я забываю,что собирался (хотел) сказать.
Since forget is often used instead of forgot or have forgotten(though some people consider it incorrect) we'd better say
Я забыл, что хотел сказать.