Welcome to the forum.
A sudden loss of power points to either a component failure or bad fuel and the consistent poor cold-start performance points directly at the glow plugs. These plugs will have a relay & fuse and either of those could have a fault too. Checking the electrical items isn't too difficult but the glow plugs are a little more difficult.
The underpowered issue is probably separate and could be the result of several things.
See if you can draw some fuel out from the bottom of the fuel tank into a clean container. Does the fuel look clear? Any signs of water in it? What about debris, like rust, sand, soil, clay? It's possible that you filled it up with contaminated fuel.
It would be worth checking the fuel & air filters.
Finally, clean the hoses around the turbocharger inlet, remove the hose and with your finger, give the fan inside a spin. It should move very smoothly (although it might resist movement slightly when cold) and the shaft shouldn't wobble at all. If that's damaged, there's the power issue - and if you don't have an intercooler, driving it risks destroying the entire engine.