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Contains spoilers for Act four:

Paul and Beth traveled back to 1999. While Paul used his knowledge of the future to gain money and power, Beth didn't really take advantage of knowing the future. She would be also able to make a fortune and thus making Monarch less powerful. She could tell Will to create two CFMs ...

She did try to change the past as she warned the government about 9/11 and was ignored. During this part of the game the main theme is that no matter how hard they try the past can't be changed.

Its an act of the timeline correcting itself. Beth can't get rich because the timeline / universe won't allow it.

Also later in the game its said that the past is set and that the only thing that can be changed is the future and the present.

Beth suffered through a long bout of depression/mental anguish after she traveled back to 1999, due to her experience surviving the End of Time where she was originally sent. The continued presence of the End of Time meant that Beth's mission to prevent it inevitably fails, which combined with additional attempts at changing future events paralyzes Beth. She did not care to better her position using her knowledge of the future because she had only known her mission to stop the End of Time, which she believed to fail, and thus everything was meaningless. Much of her thoughts are detailed in the Beth's Journal collectible from Act 4, Part 3.

Selected entries to illustrate above:

February 28th, 1999:

The End of Time.

My greatest fear was realized. I’m still trying to grasp what that means. My entire life has been devoted to stopping the Fracture. What I saw… was it proof that it couldn’t be stopped? I can’t believe that. There’s a logic to all of this, a meaning, I just haven’t found it yet. I’m not ready to write down details of the End of Time. Not yet.

September 11th, 2011:

I tried to stop it a second time. I warned them. Told them exactly
what was going to happen. Nothing changed.

I came to terms with the fact that none of this could be changed as a
child. But since then I’ve seen things that make me desperate to prove
myself wrong, desperate to believe that what I’ve seen cannot come to
pass.

I tried to stop the rise of Monarch. I tried to stop Paul Serene. Both
versions of him. Every action was negated by the force of a timeline
that cannot be bent or broken.

I’ve tried to stop tragedy as well. Jack’s parents, Christmas of 1999.
I knew what happened to them. This time I was there. Car crashed
exactly as it was supposed to. My presence only led it to happen as it
always did.

I went to the funeral. I watched Jack cry. I watched Will build up a
wall around himself. I watched two brothers change forever in that
moment, and I could do nothing about it.

And now this.

Fuck.

May 22nd, 2003:

THE END OF TIME

Been having recurring nightmares about what I saw. I’ve been dreading
doing this, but it’s time. I have to write it out. It’s the only way
to move past it.

...

I’d stepped out of the machine into the one thing I spent my entire
life trying to prevent. The Fracture was never fixed. I was living
through the proof.

The first hours were the hardest. I was living through a nightmare
that I couldn’t wake up from, surrounded by constant reminders that my
mission would fail.

...

Eventually Paul managed to get William’s time machine working. He was
convinced that he needed to travel back to the earliest possible point
in order to stop this all from happening. He had yet to learn that the
past couldn’t be changed. I followed him through the machine but he
escaped. He formed Monarch. He built an empire.

I watched the birth of a monster. I can’t escape the feeling that my
actions helped shape what he became. Maybe I could have guided him.
Maybe if I hadn’t tried to hunt him down and kill him at the End of
Time. That can’t be my part in all of this. There has to be more.

February 3rd, 2005:

So I decided to take on some hobbies. Tried poetry. Got depressed.
Dabbled with a guitar for a while. Smashed it against the wall. Then I
started experimenting with graffiti. Seems to be working out okay for
me so far. It’s fulfilling on a few different levels. Partly as a form
of self-expression, partly as a means of release, partly just to fuck
with people. What happens when Joe analyzes some graffiti on a wall
one day, builds an interpretation in his head, and then three years
later his depiction of that image comes true? Not sure, but it’s fun
to think about. I can’t change anything, but at least this gives me
the illusion of some kind of power. That illusion is what keeps me
holding on.

Sometimes I leave anonymous notes in people’s mail boxes telling them
what will happen next week. I watch their reactions from afar.
Voyeurism and graffiti. That’s my life these days.

July 4th, 2010:

The day is here. Soon I will receive word that William has completed
the countermeasure. I’m sure of it. I’ve come to accept that this
doesn’t mean we will succeed. I’ve come to accept that my mission was
a lie.

But I’m not giving up. Lie or not, I devoted my life to this cause. It
gave me meaning. It’s all I’ve ever known. Even if I’ve seen the proof
that I won’t succeed, I’m not giving up. I will dedicate every ounce
of courage I have left to seeing this through. I’ll retrieve the
countermeasure. I’ll keep it safe until 2016. I will do everything in
my power to activate it when the time comes. I will fail. But I will
fail knowing that I did everything in my power to save the world.

For years I hoped that this would be the day when I would see Jack
again. I’ve given up that hope. This mission started with me alone.
That’s how it will end.

This will be my final entry.