Old Happy: "This is how I'll always be." New Happy: "I am always evolving."

When I was a teenager, someone told me that every cell in your body replaced itself every seven years.

This filled me with awe. There was something so compelling to me about the idea that you were always changing into a new, different version of you.

I later found out that this fact is a major oversimplification. Yes, every cell in your body does replace itself over the course of your lifetime — but not every seven years and not at the same rate for every cell. Your skin cells replace every few weeks; your fat cells last for about ten years.

Recently, many scientists have focused on researching the telomere, which are a part of your chromosome. As you age, your telomeres become shorter, increasing the likelihood of developing diseases like cancer, dementia, and diabetes. What you do in your daily life influences how quickly your telomeres shorten. For example, studies have found that certain choices like what you eat and how you move can increase the length of your telomeres.

And these changes are not limited to our physical selves, either. Some researchers argue that our personalities are not at all set in stone, but a constant and flexible thing that is ever-shifting; big life events like having a child or losing a job can change your personality; and intervention studies find we can consciously shift parts of our personality.

In short: we are always changing.

This was the reason that I, as a teenager, found the idea of 'born anew every 7 years' to be compelling. It offered a lot of hope. It wasn't just that you could change, but that you were already changing. There was no need to be stuck in the past, and there was the possibility to utterly transform in the future. Perhaps you could even have a hand in that change, cultivating the best parts of you and discovering new parts that you didn't know were within.

The next time you say to yourself, "I'm just not that kind of person..." or "I'm never able to change..." remember just how untrue that really is. You're never done.