In most RTS games, there is a Rock-Paper-Scissors relationship. It's Rush/Tower/Economy.
A "Tower", a very defensive person, will focus very quickly on defensive structures, stationary defenses, and powerful, slow moving units. When a rush person attacks full force, the defense person has an advantage. Defense is naturally easier, because its on your terms. There strategy is tailor made to defeat rushes. After the battle, the rusher has nothing, but the tower has most of his defenses left. Pack up those slow moving defensive units, move them to the enemy base, unhindered thanks top the previous defeated rush, and the rusher is destroyed.
An Economy person is ready for the long haul. They build a lot of gatherers, research a lot of tech, and will be building the top tier unit in as little time as possible. A tower person prepares for an attack from the economy that never comes. Then, the tower players defenses don't look so fancy when the economy comes in with units tiers above the towers.
A Rusher builds the basic unit as fast as humanly possible, to catch the other by surprise. An economy player will have an impressive number of SCVs that don't mean anything against the endless hoard of zerglings. The economy players measly defenses might fend off the rush, but not without a crippling blow to the players base and resources. Meaning the second attack will be like taking candy from a baby.
If you have a problem with rushes, focus more on defenses. Look for slow moving units, stationary units, and AoE units, as well as whatever best counters your opponents base unit. If your just not good at that strategy, which may be likely if your having a problem, try a Rush strategy of your own. Maybe try to harass more, or look for other defensive options or weak spots to exploit. If the problem is ground only units, get fliers and harass. There is a solution. Worst case scenario, play the side you have problems with against the race you usually play, and figure out what works against you.