Spayed too early?

Sorry this might be lenghty but I'm searching for information and would
like to give as much background info as possible.
I have a 2 year old dog that was adopted from the shelter I was
volunteering for. I first fostered her for several months and began to
see signs of severe anxiety and fear of just about everything but my home
and me. Walking on leash was a terrible experience for her, she tried to
run from everything, garbage cans, mailboxes, passing strangers,
EVERYTHING. I've worked with her over the past two years and she has made
a huge turn around. Today she loves her leash and going to the dog park,
to friends houses for bar-b-ques and all the local dog friendly bars,
shops and restaurants. We've worked hard for a long time and it has paid
off, but she still exhibits some fears, mostly of strange people, noises
and places, but she seems to have developed enough confidence to overcome
these situations pretty quickly.
Her background: She was found with her mother on the streets of an urban
area, there were no other litter mates. She was about 1 month old, puppy
teeth were starting to come in and it appeared mom was beginning to wean
her by regurgitating food she foraged from garbage and such. The mother
was very ill and full of parasites, the pup was a bit healthier but still
full of parasites. It was determined that the mother would be put down,
even though the pup wasn't eating on her own. At this point the pup was
spayed and I took her home and began fostering her, hand feeding mush and
trying to get her on a good diet. I began socializing her with other dogs
and people, leash training, etc... At about 4 months of age I began to
notice intense fear and anxiety. I read books talked to vets and trainers
and bahaviorist and I adopted her, for fear of what might be her fait if
left to the flimsy adoption screening of the shelter. I knew if she was
placed in the wrong home she could likely become aggressive, through bad
training techniques, etc. or be returned which might only add to her
anxiety (besides, she stole my heart somehow, she is a very sweet girl who
really needs special attention). What I can't figure out is where the
fear is coming from. I realize much of it can be inherited from her
parents, but I've had her since she was about 1 month. Since the day I
got her she has never been mistreated in any way. I'm now begining to
wonder if it's possible that she has so much anxiety because she was
spayed so young. I know this is a bit of a controversial topic, but most
vets I've talked with recommend waiting until at least 4 to 6 months of
age. I also realize that the lake of littermates might have an effect,
but no one knows why she was the only one. I'm just wondering if anyone
out there has had any experience with a fearful dog to this degree, and
what might have lead to that fear, if not brought on by a human who was
cruel or uneducated. Thanks in advance for any input!