I have always lived like a woman running out of time. I remember thinking, as a child, about a woman’s biological clock, and how she would only have a certain amount of time if she really wanted to be a mother; I wanted to be a mother. And, I looked at the obvious realities around me, the differences in terms of the type of “attention” that younger, prettier girls seemed to get versus women of maturity. (Maturity, I thought, was the age of 40.)

Even as a teenager, I thought it was over at 40 and I worried about it! I sat under a pine tree once with my best friend, Laura Erlacher, eating Häagen-Dazs ice cream. (It was our weekly ritual, to buy a pint of coffee ice cream for each of us and hide under a tree and eat it all and talk.) On that spring afternoon, I said, “We’re 16, we really need to start working because like, this is our prime. We have to have kids, and we have to get our jobs, because when we're 40, forget it." She still laughs at me to this day for that ridiculous conversation, because she remembers how intense I was about it.

And even when I wrote my memoir nine years ago, I talked about the arc of my career and how being fired at the age of 39 was the fulfillment of what I was warning Laura about decades earlier.

Here is the excerpt, from my book "All Things At Once," in which I was realizing it in real time:

“It’s not going to work out,” Sean McManus said… It was May 2, 2006- my thirty-ninth birthday. … I said, “Why?”… Sean McManus simply shrugged and said, “It’s just subjective, I’m really sorry.” Subjective. ...and as I walked out of CBS News that day I allowed myself to think how prophetic I had been, all the way back in high school. I’d known all along that my career would end at forty. I was a year ahead of schedule.

I never felt time was on my side, and as a result I spent a lot of my life rushing against what has turned out to be a fake deadline. I had that sense of time running out for so much of my life. At 30, or 35, or 37, I couldn't have imagined having 10 minutes to just do something like rub my pig's belly and, if I would have done it, I would not have been enjoying it. I just would have been thinking of the millions of things that I had to do, and I would have been stressed out.

Much of my life, too, I struggled with with body image and, in my book "Obsessed: America's Food Addiction — and My Own," I admit to some terrible failings, like eating disorders and over-exercising. When I was in local news, I spent so much time trying to look like a young beauty queen, because beauty queens with no experience got all the jobs. I was never beauty queen, and I never could’ve been a beauty queen, but I tried.

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Somehow it always felt like it was about age and, in that generation of broadcast news, it was. I remember thinking “Wow, I am living exactly what I worried about."

That was what I believed the path for a woman was: Working really hard in my 20s and 30s, and getting great gigs, and then the minute I hit 40... done! So I didn't expect to be hosting a show like "Morning Joe," or writing books about my experiences or creating KnowYourValue.com, which is actually a highlight of my career. And personally, I love being a mom more than ever — and I’m getting married!

What I hadn’t realized was that times are changing for women.

I was wrong about the limitations I would face. What I hadn’t realized was that times are changing for women: A generation ago, a woman who was like me would perhaps be the only woman in the room. Now there are more women in positions of power.

We have gotten past the time when women were really awful to each other, playing to each other's fears or strengths perhaps because they were used to being the only ones in the room. I wrote in my book, Knowing Your Value, about my experiences with female managers who played right into my fears, because they knew how to pull emotional strings.

Today women truly see the value in each other; that has changed in the course of my lifetime. We want other women around us and we’re sticking up for each other in really honest, no-bullshit ways. It's no longer about just promising “mentorship” or “words of support,” it's about helping women negotiate contracts and get paid what they're worth; it's about getting results. That’s the way I operate: Whether I’m pushing my bosses about another NBC woman’s contract or making sure that another woman gets enough time on our show, what I do to help is real and tangible. I love spreading the Know Your Value message and I’m grateful to still be in the game so I can help as many women as possible.

Today women truly see the value in each other; that has changed in the course of my lifetime.

Every stereotype that I had in my head about age and what would be happening in my life and career, I shattered all of them. And I wasn’t trying to.

When I was fired at the age of 40, I realized that I would still wake up the next morning as a mother of two amazing girls and as a broadcaster — just, this time, with nowhere to go to broadcast. And I did not give up on being either a mother or a broadcaster: While it took a year to find my footing again, I took the risk to put myself back in the game and, because the stereotypes in reality had changed over time, there was a runway for me that I never thought would exist.

I proved myself so wrong that I don’t even know what I’m supposed to feel like at this age! Being 51, part of me still feels like, Wow, that’s really old! The other part of me is so comfortable with who I am, and so comfortable, finally, with being “in the moment.” It’s taken a long time to be able to do that, and I think my age is a huge part of it.

Weirdly, being very transparent about little tweaks I’ve done to myself and the stupidities behind them, about my weaknesses and struggles with eating disorders, and even having the president tweet about me "bleeding badly from a face-lift," has allowed me to let it all hang out.

This may sound strange, but now that I don’t care what I look like, I think I look better than ever. I feel like I exude something better than when I was 30, and that is something that I just didn't expect to be able to say at 51.

I'm in “Eff-it” territory, and it's just fabulous. It’s also awesome to have enough room in my brain to enjoy my birthday and to rub my pig's belly!

As told to THINK editor Megan Carpentier, edited and condensed for clarity.

Mika Brzezinski is the co-host of MSNBC’s “Morning Joe” and founder of “Know Your Value.”