When I'm "on", the last thing I can do is relax. I'm just too jacked up on an adrenaline-laced need to get shit done that when I sit down and try to unwind, I end up getting more frustrated because I just can't come down. More frustration and I'm even less likely to relax: the stress goes through the roof.
For me, I've found that relying on guided meditations increases the likelihood that I'll be able to relax successfully, because it takes the pressure off of me to think about how to relax: someone has done all the work for me. I highly recommend using guided relaxation sessions, especially when you're really having a hard time calming down.
I've personally listened to the following free, guided meditations and can vouch for the quality. I also selected the following because they promote many of the concepts outlined in The Relaxation Response, a classic in the science of relaxation, especially passiveness and a focus on breathing.
Try them out! I always feel so relaxed after doing any of these. Hopefully you'll find they work for you.
From Diana Winston of the UCLA's Mindful Awareness Research Center:
1. 19 minutes - Mindful meditation with instructions
2. 12 minutes - Body, breath and sound mindfulness
3. 7 minutes - Meditation for working with difficulties
4. 10 minutes - Empathy and compassion
From Duke University's Student Health Resources:
1. 19 minutes - Progressive deep muscle relaxation
2. 20 minutes - Basic meditation exercise
3. 10 minutes - Quick, relaxing meditation