Hi, I play terran so I followed what he was doing more than what you were doing. Some protoss player will probably give you some great advice on P specifics but I'll just mention a couple of things I noticed.
1: You never scouted his base. You didnt know if he expanded or not until your oracle popped out. He could have taken a gold base
2: The oracle. You got everything in place and after you created the thing, you just let it sit/hang there. The terran scanned you, saw the starport and took his time to build a turret etc. He was pretty slow so you could have hit him really hard. If you go for something like that, every second counts.
3: Upgrades. If you're going for gateways like you did, get some upgrades. In the last battle terran had 1/1 and you had 0/0 (your forges were late). At 16 minutes, most terrans will have more than 1/1
4: Watchtowers. Try to place a unit there. You would have spotted the marines walking to your third and you would have seen the two dropships coming. (or build observers)
Further: Spot your weaknesses by watching your replays. I would say you're not very good at handling drops. I hate drops. I'm not good at handling them. I keep mines and turrets near my minerals lines and if I can spare it, a thor (vs zerg) or a siegetank (vs t&p)
You've got the nexus cannon and you could build a cannon in your mineral line.
Btw, you had no detection. Four banshees would have wrecked you, or some crazy terran ghost with a nuke :-)