This is something that's good for irritability and, because it calms you down, it's good to know about if you have suicidal feelings, because if you have those feelings, being in an agitated state is not what you don't want. What you want is to be calm so you can think things through.
It's incredibly simple but effective. Dialectical Behavior Therapy, which was designed for people who are very suicidal, recommends this as one of their methods for coping in a moment of crisis. Slow breathing. You can use it in any stressful situation, just taking a few slow, deep breaths through the nose to get calm and think about what the best thing to do is. With a little practice, you can do it so nobody notices. There's loads of evidence - laboratory and clinical - that shows this works. Also, you can use a daily breathing routine recommended by psychiatrists.
Psychiatrists Richard Brown and Patricia Gerbarg recommend this simple exercise - 20 min. of this twice a day for stress, anxiety and depression. Lie or sit comfortably so you can breathe freely, with a hand on your abdomen to feel it go in and out. Breathe slowly through your nose, 5 breaths a minute, inhale about 4 sec., exhale about 4 sec. The authors have published 6 papers on breathing exercises and use them in their work.
The therapy DBT also recommends distractions for a moment of crisis. If you're having trouble with the hobbies, try this. Do a complete survey of all your entertainment things and see if there's just one or two things, like your favorite music or movie that you think you'd like. If there's one movie you'd like, watch it once or twice. Then, look for things that are like that movie. You can find lots of full-length movies on YouTube if you search with "youtube reddit movies" also episodes of TV shows and clips from shows. You can find a lot of free entertainment. YouTube has lots of audiobooks. Looks for things that are not boring, are fun or interesting. Not sad things. Keep adding to your entertainment supply to give it variety.
If you find that you're constantly thinking about your friend, try this. Reserve a special time for thinking about the person, like after dinner while you're having coffee, decaf if you have insomnia. Think any way you like during that time but when time is up, you have to go on.
I've answered a lot of depression questions, with advice from experts. There's some important things I haven't touched on here, like the basic lifestyle program developed at the Univ of Kansas and mindfulness. You're welcome to click on my name and read.
Your chance of pulling through is great. Only a very small fraction of depressed people attempt suicide.