Why are you expressing ? If you don't need to then I would leave it for a while and concentrate on getting the breastfeeding sussed ! Get something to entertain yourself - I read loads of books while feeding DS1, and watched loads of videos too.
With expressing a lot of it is in the mind - if you can relax and forget about what you are doing then you often get loads, if you stress and worry you aren't getting enough then you hardly get any.
Massage the boob - it can help a lot.
If you have lots of hands or are very adept you can pump at the same time as feeding - you usually get more then because of the stimulation of the baby. Otherwise, first thing in the morning is often a good time to get more.
Also it is better to do long slow pumps than lots of fast frantic ones. If you got breathing sorted for labour give that a go - I found concentrating on breathing often helped get a letdown.
If you really need to pump and get a lot, then maybe go for an electric pump, and double pump both sides together.
Is this your first baby ? If so just try and relax - you say you feel tense all the time. Go with the flow, follow your instincts, and believe those little people are much tougher than you think ! It really should settle down soon, and you will get into a routine that suits you both.