I used to have a lot of trouble playing against Terran until I realized that it wasn't my mechanics that were losing me the games - my macro was solid enough and my micro got the job done - it was my mindset. I played how I wanted to play. I cheesed with one base banelings, or I all-ind with roaches, or I took five bases and rallied ultras into tank lines until I ran out of money. It wasn't until recently that I figured out how you are supposed to play against good old terran.
The secret is to treat your opponent like he/she the hottest girl you know. Starcraft can essentially be boiled down into a dating sim, and your goal is to not get friendzoned and to enter into a serious relationship with this girl. Here's how this analogy pans out:
Building your economy is the same building bonds of trust. Trust is essential for a healthy relationship; you want her to trust you as much as possible before you make the jump to actually dating. In fact, a lot of high level players - and I agree with them - say that trust is the absolute most important part of starcraft/dating. But be careful! If she trusts you too much - you make too many drones - your romantic adventure will be cut short when she hugs you, says you're a really sweet guy and she still values your friendship, and then stims her marines and mercilessly tears your defenseless base to shreds.
Tech buildings and macro hatcheries are important memories and experiences you share. When you take her to a movie and build your spawning pool at 9 supply, she's going to turtle up a little. Don't be overzealous with your baneling nest/ferris wheel date. If you come on too strong, she'll throw up defenses that you'll be hard pressed to ever penetrate. And the timing of your spire/romantic dinner affects how successful it is - too early, and any real chance at taking it seriously is lost; too late, and tank fire annihilates your paltry ground army. Don't forget, however that different girls like different things. Maybe a girl who is less interested in ferris wheels than she is giant war machines might be better won over by roach infestor/getting caught in the rain without an umbrella and running back to your apartment, soaking wet and panting, with goofy smiles plastered on your faces. All these things give you a better chance of success when you finally man up and ask her to go steady. Unless you're Nestea, in which case ferris wheels are ALL you will ever need to win any girl's heart.
Now, if you scout she is an expansion up on you, it means she is dating someone else. DON'T WORRY. The game is not over; far from it. You have two options. You can: A) Harass her and show her how stupid her boyfriend is until they break up; this option is dangerous because it could cause actual animosity between you in the form of turrets, planetary fortresses, and killing you, but if it succeeds, you are in a better position than ever before, and she may even come to the realization that her old boyfriend was an asshole and immediately date you for pointing it out - if this is the case then you've succeeded! Or your second option, B), is to build up the trust between you, without mentioning her boyfriend at all, and hope that when she inevitably finds out what a terrible guy he was and comes running across the map to leap into your waiting arms, you'll have the guts to embrace her. With banelings. The danger here is that one misstep can cause catastrophic friendzonage, and when you're entire army is gone and she's weeping on your shoulder/laughing at your bleeding queens, you'll wish you'd been more aggressive. The best solution to dealing with a boyfriend is ideally a mix of the two - you build mutual trust while breaking down her current relationship. Pros are good at this; watch them for an idea.
There are a few more little things to remember before you take your new found lovin' skills to the ladder. First, remember that you can glean the advantage from even the smallest things. Baneling bombs are unexpected, but not unwelcome, flattery - compliment her a few times out of the blue, and you're on your way to a happy relationship; but do it too often and the surprise will fade and she'll scan every fucking where because it's free. Creep spread is flirty facebook chat. The more of it you do, the better you get to know her, her army comp, her favorite activities, whether she's sieged or not, and all manner of exciting little tidbits. If she drops you, it's a text message. If you haven't got much else going on, then answer it right away; but beware, as girls often disguise huge midmap pushes as seemingly meaningless questions in the back of your third; but if you leave it in your inbox unanswered for too long, it will start to eat away at the bond of trust you've built thus far.
The last and possibly most important thing to keep in mind is respect. If you can't respect a girl, you can't date her, pure and simple. Maybe you'll get lucky once or twice, but if you try to take a girl who's afraid of heights and would rather be on the ground in a tank line 50 tanks deep on a ferris wheel ride, you won't have much luck. If you are so transparent with your aggression that she doesn't even have to look for your morphing banelings, she's going to throw up bunkers and end the game early. If you're pressuring her with mutas about her male friends too much, turrets will shut you down quick.
And if you do get friendzoned or rejected, don't panic - there are plenty more fish in the sea. Just look at Code S.