I constantly do this. This is one of my biggest problems. I wish I could just live in the moment.
I find that when I'm very distracted, I'm the most at ease. For example, at work, I rarely think about all the problems with my past because if I was to start ruminating past issue too much I could cause a fatal accident. My job requires the use of powerful machines that require my complete attention.
Another thing about letting go is often we hold onto things that are not actually a big deal. For example, last night I sent out an important introductory email to a high level business professional. I made a typo, but didn't notice until after I sent it. For the entire day and night, I worried about the typo and what the repercussions would be; however, this morning, he responded, and I don't think he even noticed the typo.