Marines can effectively enter a fight stimmed without the health sacrifice. Effective defence requires micro-management. Most effective terran armies include a science vessel to wipe out annoying guardians, take out a protoss army's shields and detect dark templars, wraiths, and lurkers. Despite claims or theories to the contrary there are no unbeatable armies . There are no unfair units.
Every attack has a counter attack that can beat it.
You need to build up experience to learn how to beat any given individual attack. If you find a certain kind of attack difficult to deal with, adopt that attack yourself and see how others deal with it. However creating a successful defense or counter attack to any impending attack requires reconnaisance to know what your enemy is up to. Also remember that an experienced opponent will also be trying to observe what you are up to. A lot of players are perturbed by some of the highly skilled players who will build an army as quickly as possible and send them straight at them.