Adam Neely, NYC-based bass player and composer "When anybody tries to do something that's outside of their ability, there's a natural tendency of the body to want to tense up, which is really unfortunate because it's the exact opposite of what should be happening. We should be relaxing into the activity and relaxing into the motion."
Most classical harpists are taught some form of relaxation. The idea that, if your muscles and tendons are going to be "at work", they also have to find an "at rest" moment in order to continue. I'd had students who failed to heed this advice and have suffered longterm damage.
Adam also suggests slow practice for the student, until mastery of the passage is attained. He gives a host of great reasons why this can be very helpful, and applicable to harp as well. His video is called "The trick for playing bass fast (according to sports psychology), this is his lesson #23.
Tension can cause all kinds of stumbling blocks later.
The habit of playing with tension is deadly. It eventually catches up with you.
So, watch the video. Heed the good advice. Then go tune your harp and play something within your skill sets. Make it lovely and stay relaxed.
Tension is never your friend.
A good way to think about relaxed is to try to play gracefully. It's very difficult to be graceful when you're tense. So, not only should you make your music sound lovely, it should look lovely too.
more about staying relaxed at the harp
Here's a beginning etude to help you build relaxation into your techinque. Use the rests in version A to release tension, then, when you move on to version B, maintain the relaxation between each note that you place.