1. Download the 4GB Patch here: https://ntcore.com/4gb-patch/
   
2. Extract it from the .zip file
   
3. Run the program & point it to hl2.exe (which will be inside your steam directory, then steamapps/common/Source SDK Base/)
   
4. Click ok and it's all done. The unpatched exe will remain in that folder with a different extension to avoid conflicts

This will allow your game to have access to 4 gigabytes of memory rather than 2, and you can run your game with textures set to high again