I've been having a hell of a time vs the Terran AI as well and I've learned a few things.
The first is to get that mothership core out asap. 14 pylon, gateway as soon as you're able, get that assimilator up at 14/15 supply too if you're able (not sure about how optimal that is but it's working well enough for me) and get it out asap. Soon as the gateway finishes, put that cybercore up and the moment that is done, boost your mothership core out. Always producing zealots or stalkers (or adepts, since they do so well against marines) will be essential.
Never fight in the open unless you have a clear numbers advantage. If you're a good player (I'm not) I'm sure you could get away with skirmishing like that, but the AI tends to go hard so the defenders' advantage is awesome. Make sure you have an army up and when they hit your ramp, overcharge some choice pylons, get a guardian shield up and hit them with everything you have.
Keep reinforcing, slowly expand and ensure you have map vision so you know where the AI is going to hit. They're usually fairly predictable but I've been backdoored a few times so I'm a bit wary. Ghosts are nasty little things so having an observer on hand to spot them if they are stealthed is always lovely.
Basically though - let the AI come to you and fight on your turf. If you can beat it back and have the means to reinforce a significant amount push and make sure you have a proxy pylon to keep your warp-in cycles going. If you don't, just rest on your laurels, ensure your upgrades are up to speed and reinforce while waiting for them to hit again.
It's what I've had the most success with, so hopefully that helps.
TL;DR:
Defender's advantage
Upgrades
Mothership core out asap
Constant unit production
Do not pick a fight on neutral ground unless you're confident you can whittle them down prior or you have a numbers advantage.
I hope this helps you - I still have issues from time to time but this approach usually helps.
Also, if you can work out how to use them in tandem with your main force, Disruptors are awesome for taking out large chunks of marines, etc.