Is my sink P-trap too low?
Bring your Ptrap up or your Sanitary Tee down. 

AS noted by others you can cut the tail of the piece  where it goes into the top of the Ptrap shorter ( thereby bringing the Ptrap up ), This will allow you to have the down hill slope you need for the disposal. You may need a longer tail piece from the bell on the basin into the tee from the disposal. 

OR 

I think this is the better option because your sanitary tee is a little too high. 

You could cut your drain pipe shorter so the sanitary tee is lower. 

Use a regular Ptrap ( non flexible, there is no need for the debris catching flex connection here ).

You also need to mount your air admittance valve properly.   It  is not high enough, put it as high up as you can in the space between the bowl and the back of the cabinet. 

If you put it on a threaded connection  just above the san tee you will be able to pull it out to change it if necessary.