... I'm on the ground floor surrounded by other buildings, and am unable to attach anything to the outside of the building, do you think I would be just throwing good money after bad by buying a better antenna?
INMHO: Yes. Because, also a special 1090MHz outdoor antenna -which you can't install- needs "direct view" to the sky, if you want to use its full performance potential.
The only things helping are:
1. Clear view without any obstacles.
2. Placing the antenna as high as possible over the ground. (Which hopefully results in 1. )
For further range improvements:
3. Special 1090MHz outdoor antenna.
3a. Amplifier for the outdoor antenna.
3b. Low Loss cable like the Ecoflex 10 / Ecoflex 15.(if you have some money to waste) / Keep cable lenght as short as possible.