Hi there,
Firstly, try and relax! I know that can be easier said than done! You say you've been driving stick for 10 years, so you already know the 'how to'. Clutch Control is what's tested on a Manual gearbox driving test, so it pays to practice and get your skill at it as best as you can. That said, when I was taught to drive, I still remember word for word what my fantastic instructor told and discussed with me.
Think of clutch control as 'power control' as well. The engine makes the power and you, depending on how much you press the clutch in or let it out, control how much power translates into forward (or reverse!) motion. The more you let out, the more power is transferred to motion. On flat ground, do as many standing stops to motion as possible. Each car is different, but that 'biting' point is what you need to get used to. Clutch fully in and the 'power isnt connected' bring the clutch up, and up and you'll feel the 'biting point' - now you know, release the handbrake, give a 'little more gas' and 'control' the clutch a little more and it'll move forwards. Practice that, and perhaps think, 'I'm Controlling the car/clutch/moving forward' Once you get better and better, apply the same technique to the Hill start. You'll basically get to biting point, and will need to add more gas to prevent stalling. The rest will be the same as what you've practiced as on the flat starts.
Another technique thatI used to teach my younger brother and my niece clutch control is to say out loud each and every step as you do them. "Depress clutch, selct first, check mirror, clear? , add a little gas, bring clutch up and up and up, hit biting point, add a little more gas, release handbrake as I control the clutch some more so I get forward motion.
Try it, it may help!
Lastly, relax!!!
Cheers, DtM! West London & Slough UK!