I've found a particularly helpful breathing exercise that's been helping me calm down and improve sleep quality. I am now trying to do this nightly and find it works better on me than many breathing exercises. Many people have said that yoga and such helps them so this may also be beneficial.
1) Set timer to 20 minutes
2) Get in a relaxing position (I like cross legged upright)
3) Right when you press start hold your breath until the timer (counting up) gets to as close to 1 minute as you can.
4) Exhale, breath normally and rub your temples for 30 seconds up until 1:30.
5) From 1:30 to 2:00 stop rubbing temples and just breath normally.
6) At 2:00 take another deep breath and hold it until 3:00, as close as you can get.
You should be able to hold for a minute the longer you do this
Just repeat this process for 20 minutes and it really relaxes you and is supposed to stimulate vagus nerve (releasing antiinflammatory messages). I find it really helps me sleep and calms me down.