Aw, sympathies, I was were you are 8 years ago (DS2, delivered at 31+4, in SCBU for 5 weeks, never actually got him on the breast so exclusively expressed for about 5 months).
First off, it sounds like the two of you are doing great 
, she is afterall gaining weight.
Secondly, how much milk you gain when expressing says very little about your actual supply. Lots of woman just don't get big yields from expressing, but do absolutely fine when feeding a baby.
Your LO is still v young and tiny (I presume) so if you stick with it, things are likely to improve with time.
In the meantime, focus your efforts on encouraging let-down:
As Teaandhobnobs said, try and relax (easier said than done, I know) - a 'babymoon' can be really good if you can manage that. Take to your bad with DD, snooze, cuddle, read, watch the telly or whatever with lots of skin to skin contact and very frequent feeding.
Express from one breast whilst feeding from the other IYKWIM - it requires lots of cushions/practive/the arms of an octopus, but it was the only thing that gave me increased yields with DS3 (a whole other story).
I also found a photo/worn baby grow to sniff usefull to trigger a let-down.
Breast massage (medium strokes from outside of breast to nipple, all round the circumference of the breast) helpful before an expression session.
Also, warm get pad in bra before expressing seemed to help.
Express at night time, ideally twice.
I took the same supplement as you and found it helped more than Domperidone, but responses are of course different from personn to person.
Remember it is milk removal that stimulates milk production.
Sorry, 'tis a bit of a ramble.
Have a look at Kellymom website as well - full of good advice, equally my personal 'hero' Dr Jack Newman.