You probably need to work on your soldering skills. This isn't meant to be a put down, but Badwrench is right--that board is in bad shape from too much heat, for too much time, and is covered in burnt flux. Putting things together on a clean board is the easy part. Reworking already soldered parts is much harder.
If I'm guessing what you're doing, you're thing to connect the wire to the pad at the end of the diode. I'm also going to guess that the diode was soldered by the manufacturer. What they did was use a solder paste, applied *exactly* the amount needed using a stencil, and then baked the whole board. The controlled amount of solder, the fact that it might be lead free, and the predictable burn off of flux means that getting the joint to flow again isn't easy. And the solution is *NOT* more heat.
When redoing soldered joints you really should first apply a little bit of flux. Soldering flux is rosin based (NOT the acid flux plumbers use for brazing pipes, which will also sometimes be called soldering flux) and is applied with a toothpick or a brush. Your solder in the spool has flux in it, that flux vaporizes when soldering. You need to put flux back to help the solder flow again. Some people forgo the flux and just use a little bit more solder (thereby adding flux from the extra solder) but that's a waste of solder and the last thing in the world you need is to put more solder on that board.