This is my second one and I went back to work at 3 mos, and I have to pump during the day at work for his feeding the following day.
I've found if I am stressed I just can't seem to pump much, so as silly as it sounds you need to try to take a couple of deep breaths and relax. The more worked up you get before you pump the worse it will be. You can try taking a hot shower before you pump to loosen things up. Then when you're pumping, try having a lot of pictures of LO visible and really focus on the photos/zone out and mentally just think about holding your LO and nursing. I also find when it starts to come out if I gently massage my breast ( from the back of the breast towards the nipple, don't massage the other direction ) it can help with a let down.