My 4-month yellow lab is nipping

[stacey]

Gussie, my 4-month old yellow Lab, can't seem to engage with people
for more than a few minutes without lunging from the ground, mouth
open, and biting. Sometimes, she nips at the hand that pets her,
including mine, At just 33 pounds, she has bitten hard enough to
bruise and cut me and tear clothing. Her trainer said to spray her in
the face with water that is one-third lemon juice, a Citronella spray,
or Bitter Apple--all of which are reportedly not harmful to her but
act as a deterrent. My problem is, I don't, every minute of the day,
have one of these by my side. I might be carrying groceries, getting
the mail et al. She has a lot of rawhide and chew toys. I walk her
several times a day and otherwise spend a lot of time with her. I love
her very much but she is hurting me and I am afraid may bite a child--
she's already bitten a couple of teenagers. I don't believe in
striking any living thing, Here's what I have tried (not all at once,
of course, with the exception of positive reinforcement, a mainstay of
her training) in response to the serious biting:

---shouting ``Ouch! You hurt me!'' to startle her

--Spraying my hands, legs with Bitter Apple, putting a squirt in her
mouth

--the water/lemon juice spritz in her face

---holding her down at ground level in an effort to stop the lunging/
jumping

--LOTS of positive reinforcement (including treats) for good behavior

Any ideas? Thank you very much.