Can stress decrease my breast milk supply? What should I do?

Yes, most definitely. I struggled so much and was only yielding 30 to 50ml (3 weeks post delivery) every 2 hours and it got lesser and lesser each day. Instead, what I should have done was to relax and think of happier times. Be near to your baby (if he's sleeping) and get your spouse to cuddle you if he's available. Otherwise, switch on your favourite drama and have a hot drink and biscuit while you pump.