I'll do a bullet point of things that went wrong that game:

1. You lost because you attacked into a prepared concave with a smaller force. Your colossi are far less effective when the enemy has his units really spread out because your hitting less. Furthermore, when the enemy is spread into a large concave like that its extremely powerful. Basically the terran player was just able to surround your army.

2. You should have taken a third as soon as you held off that drop you waited way to long to your third and were basically all in with that attack you were doing.

3. You need to tech up to storm faster templar and storm are so extremely powerful. You also need to get blink out sooner too.

4.You don't really want that many stalker in your composition they are not that good in the army. You want more zealots which are better at tanking damage and keeping units away from your colossi. you want just enough stalkers to take down vikings and that it.

5. As protoss especially when opening colossi you need to know when to push and how to be patient. When you pushed you had no third didn't have a real good idea on what the Terran player was doing, didn't have a good deathball yet, and didn't take into account that this was a really open map which helps the terran.

6. You need about 3 observers one towards their base and 2 to spot drops on either side of your base.

7. If opening Colossis get your collossi up keep just enough stalkers to defend drops and only warp in zealots. Tech up to templar archives fast and then start adding in archons and about 4 high templar for storm no less. This will make taking care of vikings so much easier throw down 2 storm (spread out not on top of each other) then focus them down with the stalkers and just let your collossi and zealots do their work. Also through down another couple storms on their army and you should be able to walk through lower level play easy this way as long as you can defend drop play.