Contrary to meme belief, Terrans are the rush race. They have the highest win rate in the 5-15 minute period and have the lowest win rate in the 25+ minute period. You must win early with Terran otherwise you will be beaten by superior units.
Terran is all about mobility and the weakest Terran units are the ones that are the least mobile. Siege Tanks are strong but only when sieged, and their mobility is a huge negative. It forces a strong mobile force (medivacs, hellions, stimpack) to become stationary. When you start playing the positional game against Zerg, who have endless units, you're going to lose. You need to play fast and aggressively against them and must win before the 25 minute mark.
If you're having problems winning before the 25 minute mark it could be that you're focusing too much on a mid game effort instead of an early game effort. If you see the early third, you've just gained a mineral and time advantage against him. Attack him, he won't have anything. If you don't see a third expansion, attack anyway. Hellions rock. Bunkers rock. Everything rocks. Contain and gear up for an expansion, soon your mobility will come in to play.
Get a medivac and load it up with marines. Send it to the back of his base. At the same time, have an army of pretty much anything at the front of his base and attack. With your attacks always send 2-3 SCV's and send them in the front. The AI will always attack these first. You can use this to your advantage in a number of ways. Alternatively you can have one SCV lead and two heal so he lasts a little longer.
If at any time you lose your position, just sell the bunker. You lose 25 minerals, it's a non-issue. Bunkers are amazing, build tons of them.
Marines and Mauraders are amazing units until the enemy gets infestors. Generally at that point you're fighting an uphill battle since your strength, mobility, is up shit creek.
You must win early, either through ruining their economy or outright winning. You must not let them reach that point where they win. The only reason Protoss stands a chance against Zerg in the late game is because of the archon toilet, otherwise brood lords and the zergs insane ability to tech switch (the real strength of the zerg) wins every single time.
Focus on unit preservation as well. Terran builds units slower than the other races. You must preserve every last unit.