How old is your gf? This got nothing to do with age actually but
it seems that this is rather common with younger girls than older
girls.
Anyhow, it's rather common for anyone to answer "will let you know
later." "will confirm with you later." etc. especially when the
followings applies
1. He or her does not have the ability to draft out a
schedule
2. If a schedule is work out, but the mind is not installed with
"stick to schedule." program.
3. He/ She doesn't place you as someone he/she will be dying to see
more often. You get frustrated because you want to meet up more
often and his/her way of scheduling is affecting yours badly.
4. His/ her profession requires her to commit her 24 hours to her
client.
Regardless, take my advice and relax a little. This is what you
should try doing...
1. Do not initial dating that has activities that is 'yours'
interest oriented.
2. Help her out to run some of her errands.
3. Reduce the frequency to meet up and let her initial some times.
The logic is this... you will not be missed if you are always
there, but will always be there if she wants/needs you to.
4. Always remember that relationship is not about getting even with
one another if they don't like each other's behaviour but rather
learning to adapt to each other's bad habits and slowly find ways
to accept it.
Afterall, if you only fall in love due to her looks, her character
will not really matter. However, if you fall in love with her for
what she is.. you are actually falling in love with her good and
bad habits. Try not to manipulate her to change her bad habit for
you as you won't like it if she does the same. Rather, be patience
and let her and yourself to adapt each other's lifestyle.
Good luck. 
