Happiness is temporary, you must keep working to achieve a level of satisfaction from activities or events. It's the carrot that dangles in front of us our whole lives but we shall never truly reach it. Life is experience. Happiness comes in moments. Those moments when we learn to value and treasure; for those are the memories of hope when life fails us. Selfishness is doing what's best for yourself, which includes taking care of people who make your life better by being around them. It brings you pleasure to give to them because they make you happy when they are happy. It pretty much is selfishness. Almost all human actions can be traced to selfishness.
Hedonic over-indulgence isn't selfish, it damages you. Victimization of others isn't selfish, it puts you in a lot of danger and messes with your psychological well being. The idea that everything (or almost everything) is selfish requires a very superficial and short ranged idea of what's good for you. Real selfishness has to include long term consequences, full contexts, and a broad perspective. Risking or even giving your life can be selfish, when a person is so important to you that a life worth living isn't possible to you anymore if they die and doing so prevents that. Because selfishness is measured by quality of life, not mere survival. When no quality of life is left to you, living becomes un-selfish, especially if you know that you could have prevented it but didn't. If someone is not an enormous personal value then this wouldn't apply. If they are a disvalue, or a threat to you or your values, then their death isn't anything to care about. The same holds true when you are so attached to someone that enduring their death would leave you in a constant state of suffering.
Perfection is a standard that doesn't exist. A mere fantasy imaginary state. Perfection is Zeno's tortoise paradox; something you chase, but never catch. In logic theory, perfection is a state that satisfies all predetermined conditions. So you can achieve a perfection by achieving said conditions. A perfect score on a test, a perfect party, etc.
The problem with stressful situations is that most of them are non life-threatening, yet when the stress hormone is released, instincts kick in, and our bodies respond as if we're in danger. So, to constantly be stressed puts a lot of ware on the body. It's your body, you've got to learn how to connect with it so that you're in control. In psychology it's called learned behavioral responses, which most people spend a lifetime trying to undo. It's just a matter of literally learning how to rewire your brain.