What to do with a 8 week-old fussy baby?

My daughter is 8 weeks old and has been a really fussy baby. She is still fed and sleeps on demand (every ~3 hrs during the day and ~4 hrs between 11pm-7am). she sleeps ok during the night and before noon, but she pretty much spends all her waking hrs between noon and 10pm crying whenever she is awake. She eats ok every meal, then I'd burp her, change her diaper... and then she'd start crying. I tried putting her in the bouncer, hold her at different positions, jiggling, singing, etc. --- nothing works. the crying would continue until she grows tired and takes a 30 min nap until the next feeding. It is tough for me cos I have a 2.5yr old as well and she gets jealous sometimes. It is also very exhausting for me. I'm lucky to have my parents' help now, but eventually we'll be on our own. I don't know what to do... please help