I think the solution is
SENTENCE.
Title: This riddle's solution may take you years to complete!
You can be sentenced to prison for years.
You've seen me your whole life.
Even this riddle is full of sentences.
A bar man may pass me,
Not a barman in a pub, but a man of the Bar - a barrister, or judge!
A woman paid to tell lies may create me,
A journalist, politician, lawyer, JK Rowling, etc. will indeed create sentences in their writing or speeches.
And a reverend might mistake me for a silver coin.
Sentence -> ten cents, as a spoonerism? (thanks @as4s4hetic)
Sometimes I am finished in just a breath;
A short sentence like this one can be spoken in a single breath.
Sometimes I will last until you breathe your last.
A sentence to life imprisonment.
I can, with two letters, ask you to leave.
"Go!" is a sentence.
Or I can simply order that you be sent away.
This is probably a prison sentence again, although "Take them away!" is also a sentence.
I may be words of love or affirmation,
"I love you." and "Yes." are sentences.
Or I may condemn you to isolation, regret, and despair.
Prison sentence again.
I am, to some, a matter of life or death.
Sentenced to execution.
Consider my riddle well.
The answer is closer than you think —
if you know where to look and what to look for!
Again, the riddle itself is full of sentences.
Feedback section
This riddle had some nice misdirection, including the "bar man" and the line with "with two letters" which would make most solvers think of letterplay. This line, in fact, had me hung up for a while on TOUT as a possible solution (add two letters to get "GET OUT"). The giveaway lines for me were the "love and affirmation" and "isolation, regret, and despair". These put me in mind of an old joke I dislike:
"I am" is one of the shortest sentences in English, but what is the longest?
"I do."
With this in mind, the last verse of the riddle clearly fitted well, and when I went back up to check the rest, most of it fell into place quite quickly. The "woman paid to tell lies" and "reverend" lines were the only ones that weren't immediately confirmatory for me.
