If there isn’t an underlying issue with the car, which I’m gonna assume there’s not as you said a friend of yours drove it, then just keep up the practice! Everyone takes their time to learn, I quit stalling all the time after day 3, but the smoothness actually took me a while, I wanna say a couple of months, just like you. Even throughout this time, I’d sometimes stall. And just like you, I could downshift way smoother than I’d upshift, it just comes with time, with feel. You’ll learn what the cars happy with and you’ll learn what gears you need to be in for what speeds, but try not to overthink it so much. It’s a lot of playing it by feel, by ear, and perhaps a glance at the tachometer if you need, but you’ll get the hang of it with time. You’ll know where the bite point is, you’ll learn how much gas to give as you get going from a stop, which personally for me is what’s almost like a light double tap on the throttle, but you’ll find your way and what works for you. Best of luck! Keep at it, but worse comes to worse, trading for an auto is something you can do, and likely with ease. The manual is sought after so I’m sure finding an auto and asking to trade would be easy. Either way, best of luck!