When a relationship hits a rough patch, it can be easy to think pessimistically. Thinking optimistically isn't about rainbows and unicorns, rather, it's about being realistic. Optimistic thinkers are able to identify solutions that haven't yet been tried. In addition, optimistic thinkers zero in on what they can control, influence or leverage.
Thinking about the consequences of failure, ignoring your feelings and engaging in detailed planning don't sound as fun as advice like "Follow your passion!" or "Stay positive!" But they have the decided advantage of actually being effective -- which, as it happens, is exactly what you'll be if you use them.