How can i increase my breast milk supply?

My baby was born early at 28 weeks and is in NICU. I was able to stimulate breasts to get milk by using a pump and was getting around 50/60 ml every 3 hrs. However over the last 3 weeks this has dwindled down to 10ml ! I am still using the pump every 3 hrs for around 15 mins per breast. I drink plenty of fluids and have followed every instruction (look at baby's photo, imagine my baby, massage breasts, try not to get stressed etc) I even started pumping every 2 hours but this made it worse! I am now getting stressed as my baby needs my milk! Please help. I am trying to rest but with trips to and from the hospital and pumping sessions it isn't easy. Should i stop and wait to see if if they fill up? Im worried that if i do i wont get anything.