Nope, you're not doing anything wrong. Those fittings probably haven't been disturbed since the car left the factory and are extremely difficult to remove. First, you will need the correct metric size flare nut wrench to work on these.
If you're lucky, you can sometimes grip the T fitting with vice grips and use another pair of vice grips to unscrew the stubborn hard line if the wrench has rounded the fitting shoulders off.
You're likely going to need to remove the 'T' fitting from the car in order to unscrew the old line(s), with the fitting in a vice so you can work at it. This of course assumes that the in-line from the master cylinder fitting can be removed from the T fitting.
Worse case scenario is you'll have to cut off the line at the fitting (or further up toward the front), do a proper metric double flare (requires a flaring kit), and buy a junction fitting to go between the T and this point. You can buy new hard line in various lengths with fittings already on them and the flares done too at your local auto supply house.

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1986 560SL
2002 Toyota Camry
1993 Lexus