A320
Flight Crew Air Transport Pilot (ATP)
Fatigue; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
We were on a vector to intercept the localizer. I armed the localizer; which would have allowed everything to work out just fine. But; since we were 25 NM out; I thought it would have been better to capture the course in NAV mode. After I did that; the FMGC showed no NAV intercept. Then; I attempted to correct the problem by selecting Direct to a fix on the approach. Finally; I just rearmed the localizer and turned in with the heading bug to intercept the course. By then we were flying through the localizer. I clicked off the autopilot and turned in manually toward the course. The Captain grabbed his control side-stick to turn in more aggressively to re-intercept the localizer because we had gone far enough through that we approached the center line for [a parallel runway]. The whole time this was going on; I never looked outside to get a better situational awareness. We finally got back on the localizer for a stabilized approach. I have not been sleeping well. That is why I was tired prior to the start of the duty period. Furthermore; my marriage has not been going well. That is why I froze up for a couple of seconds and did not take the right action when it needed to be taken.  If I had only armed the localized and left it alone and used heading on the FCU to turn inward a mile before intercept; then none of this would have happened. This was due to a combination of over-thinking something and then being to slow to react when it went wrong.
A320 First Officer reported a course deviation on ILS arrival that was caused at least in part by fatigue and personal problems.