Focus isn't something that just happens because you want to focus. Saying your solution is to focus on racing is like saying your solution is to have better technique (ok maybe not quite that general...)
Do you do any mental or focus training? What is your process that enables you to focus better or focus more when you really need it?
Good call. I was meditating on how to build that focus.
I have a few separate tools to maintain it already. It's one of the three or four things I choose to keep in mind before a race. I also use the word 'focus' as a mantra when I can tell I'm losing that focus.
In training I try to use intervals as times to stay 100% focused as more is fatiguing. During the two or three controls of each interval I attempt to maintain a high level of concentration.
So when I say focus on racing I mean that I don't need to think about what I'll make for dinner that week, or what my coworker said, or if that girl remembered me. I just need to think about what will take me quickly to the next control.
Any ideas on how to improve on it?
It sounds like there are two issues at play.
1. Just keeping your mental focus stamina. I'm not an expert in this by any means but the concept of using intervals to stay 100% focused is good. Just make sure you also work on gradually extending the length you can stay focused for.
2. The other issue - which I think ultimately is the greater issue and the one where you're more likely to be able to bring about a change is how you react to specific situations in an orienteering race.
List the reasons / situations where you lose focus - passing someone, being passed by someone, when you just made a mistake, seeing something wrong with the map, etc. For each situation you should figure out the thought process that leads to the loss of focus and how you react or don't react to the thought process that you want to change. Once you identify the process and reaction or lack of reaction that ultimately is the problem you can go about changing your behaviour.
The next step is figure out what the appropriate behaviour is. Often times this will be a combination of breathing control, relaxation, and focus words leading to a specific action. You'll need to experiment and figure out what works for you an example might be that when you see someone else on your course you immediately (before you get seriously distracted), slow down significantly for 5 steps, take a deep breath and relax then remind yourself to visualize the next part of the map in extra detail. This will pull your thoughts away from the other person and because you have something specific to focus on you won't focus on racing them or paying attention to where they are going. Furthermore by visualising the upcoming terrain very carefully you will be able to out navigate your competitor going into the next control.
The reaction might be different depending on the circumstances and it won't come automatically. You need to practice these behaviours religiously to make them come super easily when you need them just like you would any specific technical skill.
Have you read any sport psych books?
I have read two or three. What you recommend makes perfect sense. Several people I train with keep an errors diary to describe the conditions surrounding the mistakes they make. Perhaps that would be beneficial in this case.