It's the nature of the mind to wander, especially when we're under stress.
U Tejanyia asks, "Is it better to have a mind clear of thinking or a mind filled with thinking?" Of course, most people say they want a clear mind clear.
He replies, "So, when the mind has thoughts, that feels like a problem. The Pali word for mind (citta) means 'That which knows and that which thinks.' So if you want to have no thoughts, you have to get rid of your mind.” Good luck with that.
In meditation, mindfulness alone is not enough. We also need Wise View – we need to understand what we're seeing. Wise View is not a perspective as much as it's a way of viewing – seeing without the distortion of desire, aversion or confusion.
If we want the mind to be quiet and there are lots of thoughts, then we are looking through the distortions of greed and/or aversion – wanting it to be different than it is. However, if we are fine with the mind being quiet or busy, then when it's quiet, we see that's it's quiet, and when its busy, we see it as busy. No problem.
Of course, if we get involved in the content of a busy mind, we can get pulled out of the present and get lost in entangling thoughts. But if we just let the thoughts wander where they will without fighting them or pushing them away – simply understanding that the mind is busy – then it's not a problem. As Suzuki Rosi said, "The best way to control a cow is to put it in a large pasture." So set those thoughts free.
And here's the magic part: if we aren't trying to control the cows (control the mind) then they will naturally start to slow down. If they have a lot of energy, they may be frisky for a while. But since we aren't pushing or pulling them, we aren't adding more energy to them. They will naturally run out energy in time and calm down all by themselves.
If our external life is busy or stressful, that stress may continue to run for a while in meditation (big surprise!). But by seeing and accepting the mind as it is without desire, aversion, or confusion, we aren't adding more strain to it. We six-R: Recognize what's going on, Release it, Relax, Re-smile, Return to sending out metta, and Repeat as needed. Or if you prefer "S"s: See what's going on, Set it free, Soften, Smile, Send out uplift.