Help! Can't letdown for the pump

My girl is 9 weeks old and I'm going back to work today. I've been pumping with a electric Medela pump for a few weeks to build a freezer stash. I've been getting 1-3 oz per breast at each 10 minute session while also nursing on demand. I bought a Medela manual pump to use at work and have been trying it out over the weekend with no luck! I can pump for 30 minutes and literally get 4 drops. I am feeling very full so I know the milk is there but I can't relax myself enough for letdown to happen. I'm very stressed about returning to work so I know that's part of the problem but now I'm stressing even more thinking about not being able to pump during my 9 hr. shift. HELP!