Why does my chard taste bitter when I cook it?
Your chard most likely tastes bitter because chard is bitter.  You can't really change that with cooking technique alone, but you can:


Use older chard, which tends to be markedly less bitter than the young chard you're using;
Avoid bringing out the bitterness, by cooking at lower heat;
Mute any remaining bitterness with salt, which is pretty common for leafy greens.