I have always said it - in the past. Often used when you are trying to prove you are right or you knew something first. Example: Person 1: “John and Mary are a good couple.” Person 2: “I have always said it.” I am always saying it - continue to say something because it hasn’t happened yet. Example: Person 1: “John and Mary should date, they would make a good couple.” Person 2: “I know, I’m always saying that.”

do you mean " I am always saying it" is used in negative things?

One is past tense and the other is present tense. So, "I think Donald Trump is a bad for America!" - you might say, "Yes, I have always said that" meaning you have been saying it for some time, perhaps even before he was elected, but you haven't necessarily said it recently. Now if I say, "I think Donald Trump is a fantastic president!" and you say, "Yes, I am always saying that!" you probably mean that you keep seeing negative things written about him but every day - including today - you disagree with them. Does that help?

en que la primera se entiende como en pasado y la otra se entiende como en presente, pero que lo dice continuamente a diferencia de al primera que se entiende como si lo hubiese dicho una sola vez