Just because we are fallible doesn't mean we are always wrong, or can never be right or sure of anything. Obviously you can be 100% certain of facts about yourself; your name, favorite color, ability to do triple integrals, etc. People of the view I just described would attack virtually any knowledge that falls outside this boundary, reducing it to "beliefs" that may or may not be true, but that in any case we can't be certain of. We can't know anything outside ourselves with absolute certainty, they say.
In response to this, I'd like to introduce a term called "effective certainty". I don't know for certain that the basic axioms of logic that underpin math, philosophy, and common sense are absolutely true, but as virtually every conscious moment of my life has served to reinforce those axioms, I can be so close to absolutely certain of their truth that it's not worthwhile to consider that they could be wrong. I can consider them to be unquestionably true unless someone brings them into question to me. I am "effectively certain" of their truth, in that doubt of their truth does not influence my life or thoughts in any way.
The same is true for the actual, objective existence of the physical world. If I really am just a brain in a jar with electrodes or something, how would I possibly know that or do anything about it? I definitely wouldn't go around sticking my finger in outlets trying to escape such a remote possibility; it makes no difference whatsoever in how I live. By being effectively certain of these things we get science. The theories of science are also examples; we don't know for certain that gravity always works as Newton/Einstein predict, but as we have no counterexamples or -evidence, it's not really worth it to question them except in singular cases.
So, we need not let our lack of absolute certainty stop us from trying to find absolute truth. As I already proved, it is out there, and though we are fallible and can be wrong, we can also be right, and be effectively certain of this. Of course, with many things, like a new theory or idea, it's important to have some doubt and be open to the possibility that we could be wrong. I'm only trying to establish that we can be effectively certain of a good deal of things outside our own existence and rely on these truths in our lives.
One last note: I wouldn't include faith in God in this category of effective certainty. Faith necessarily involves somehow leaping over the limit of where reason can bring us. It is wholeheartedly believing with somehow insufficient evidence to make a purely rational conclusion, whereas effective certainty must be built on mountains of evidence. However, doubt is still damaging to faith, so we should try not to let it affect us, living on it the same way we would with effective certainty. Faith (in the Christian God, at least) is also a relationship besides just belief of facts, further distinguishing it from effective certainty.