The Most Important Teaching: Don't Resist Pain, Accept it
The most important teaching I ever received is: when you experience pain, you should never resist it, you should instead accept it.
Think about any mental issues you might had: anxiety, depression, anger, jealousy, trauma... Or any physical problems: itchiness, headaches, cramps, tensions in your chest, your back... Resisting that pain actually makes the pain more intense. But if you open yourself to that pain without judgement, it slowly goes away. It's like dealing with a crying baby. If you get angry and frustrated, it becomes more tense and cries even more. But if you treat the baby with acceptance, gently rocking the baby in your arms with compassion, the baby soon stops crying.
The art of not resisting pain is a teaching known thousands of years ago by sacred religions like Buddhism and Hinduism, and has just recently been validated by medical science (I personally learnt it from the books of Tara Brach and the commentaries of wisdom texts by Eknath Easwaran). From mindfulness meditation taught in Buddhism (see this blog post for my recommended meditation), you are told to note the sensations of your body, triggered by negative emotions, to open yourself up to those feelings without resistance. This sounds philosophically in contrast to prayer, which seeks to "change" your mental state instead of accepting it. Prayer is wonderful in activating positive emotions that'd be natural to everyone if they weren't plagued by any mental issues. Though a lot of times negative emotions (especially if they're deep-rooted like anxiety of depression) can be so overwhelming that trying to be positive when you're drowning in sorrow is just too difficult. In that case, meditation to accept and reduce the intensity of emotions first is more useful.
How you treat yourself is how you treat others. Because you treat your negative emotions with compassion, you will treat negative people with compassion. You will be less disturbed by annoying people, you're less likely to say something hurtful to anyone who upsets you. Any sudden change in routine, distraction or tragic event, you will handle with poise. You will complain less about things you can't control and work on the things you can control, and therefore spend a longer time on things that will develop you to your fullest potential.
There is even a higher state that can be reached, as described in the excellent book "The Art of Learning". In the book, Josh Waitzkin, an International Master chess player and World Champion at Tai Chi Chuan, talks about his method in dealing with: distracting noises, catchy music that appears in his mind, unfair calls by referees, unfair behavior by opponents (he mentions how he'd be in deep concentration calculating a chess move, and his opponent would suddenly kick him on the leg).
Josh not only accepts these distractions, but whenever they inevitably occur again, uses these annoyances to activate deeper states of focus and consciousness. He'd literally practice chess moves in his house with loud music playing in the background.
Isn't that beautiful? To not only be able to handle any negative situations or emotions, but use any inevitable problem that arises in your life to make you even better? To turn something that once burdened, into something that enhances you?
Pain? What is Pain? Do not avoid the fire, you are the fire.