I am sorry to hear you are feeling stressed out Kitty. I don't think you should feel you need to be "nice all the time." We need to express when we are feeling down, angry, sad, annoyed. But it is HOW we choose to express them that counts. Do you have someone close to you that you can have a venting session with?
I think Frank here is right, that being nice can create positive feelings. I definitely think surrounding yourself with positive things can help. Like reading the Positivity Shrine on here, Watching a comedy, anything that can lift your spirits can help with stress. But when it comes to having trouble " being nice all the time" it doesn't need to be all or nothing.
Showing compassion and kindness to others, can coincide with feeling stressed and irritable. We just have to choose how we let those emotions out. Kindness doesn't have to be this constant feeling of happiness and joy. You are allowed to have bad days. If you equate kindness with being smiley all the time and you find that hard, then try re-framing it as, " I am going to be compassionate to others today" Compassion can just be consideration. Not a bubbly happy smiley response. I think sometimes, we feel expected to be overly joyful all the time. Not at all! I think it's important to recognize how we feel and own it. Which you are doing!
Here are a few tips that help me when I am feeling stressed or about to say something I shouldn't;
-BREATH. Take a deep breath. Hold for three seconds and breath out ( repeat)
-WRITE IN A JOURNAL. Put down on paper how you are feeling. When done, write 5 things you are grateful for underneath it. By channeling your stress to paper then writing something positive, can change your stress level and how you respond to people.
-ALLOW YOURSELF TO FEEL STRESSED. Give yourself permission to feel stressed. Sit with it for a second. If it helps, picture your thoughts as clouds that are just passing by. You're feeling their affect, but you're watching them move on by like on a windy day.
-SCREAM IN A PILLOW. Seriously! This can help release pent up stress and emotions. Better to scream into a pillow then at a friend!
-SMILE IN THE MIRROR. Stand in the mirror and hold a smile for a few minutes. Their is research to show that we don't feel happy then smile, we smile and then FEEL happy! It's a physiological response. It sounds really silly but it works!
-COLORING. My all time favorite. If you like art, They now have coloring books for teens and adults now. They are all the rage right now. They help induce relaxation by focusing on a task where we aren't thinking as much.
I probably gave you too many ideas here. But hopefully some of them may be helpful for you.
Let me know how it goes!