In writing, as in life, there are always periods of tranquility and periods of suffering. The tranquility part is awesome; there's not much to talk about there, just enjoy it. The suffering, though, it just sucks. But there IS something to talk about there.
The standard advice is to work through the struggle. Just keep trying and you'll get over and get past whatever your problems. This is good advice, but it's not absolute. Sometimes the best thing you can do is get away from it all.
Take a vacation from your frustrations. It doesn't need to be for weeks at a time; just two or three days is usually what you need to clear your head, sort things out, and get back on track.
A period of respite can do wonders for the mind. Don't overdo them, but don't be afraid to take them when you get overwhelmed.