When speaking with a person with a mental illness, it important to recognize that they are not using the same reference scales. Something you feel they minimize might just never get stressed as important to them. They might get excited or upset over trivial things like word choice or simple facial expressions. Awareness of the dissimilarity of expectations, awareness, and reactions can reduce or eliminate the stress involved in any type of relationship.
Memory problems are a hot-button topic in many romantic relationships. Often one finds that having to remind someone of everything can be exhausting. If you manage your expectations, you can make that process painless. Instead of expecting the other to remember, expect that they need reminding and work the reminders into a normal conversation. The tension comes entirely from feeling marginalized, when in fact the other may just have limitations on how much they can retain in a given period of time. Not being aware of the situations of others should not imply that someone is self-centered. For the mentally ill, they may not have the capacity to remember details about the lives of others, or even details of their own lives. Understanding this can eliminate many trivial arguments.
You might have a friend who is always running a bit late. Mental illness can change the way someone perceives time. It may manifest itself as a disconnection with the passage of time or even an inability to estimate how long a series of actions will take. Most commonly someone will make a reasonable estimate but forget to include any number of tasks, or simply be distracted many times and lose focus. The simplest solution is to simply give them an earlier time than necessary and then being late is still on time for you. A more personal solution would be a minor intervention, helping that person envision the whole series of tasks to get ready as one process, almost a gesture.
Habit is the most helpful and crippling tool imaginable. A good habit can make something happen quickly and smoothly, while a bad habit can derail the train just before every station. Repetition is the key to forming and breaking habits. Just telling someone to put their used dinner plate in the washer after they eat is ineffective compared to getting them up, having them gather their dishes, then walk with them to the dishwasher and show them where to put the soiled plate. The key difference is that the second method involves experiencing the process, bypassing many weaknesses in their learning process. Do this for a dozen meals or so, and chances are you will never have to mention it again, and if you try to take their dish they will react negatively, because they want to put their dish away on their own.
The key to all of this is recognizing the different scales of ability for memory, function, and awareness. Applying your ability level as a baseline for someone else can be unfair, and when dealing with mental illness the effect is much more pronounced. Often you will make that person feel inferior, unworthy, and incapable just by how you phrase a comment. Discovering the true range of the other’s abilities can save you from many argument or even salvage a failing marriage. Since many people with mental illnesses are never diagnosed, it is up to you to perceive the tension and adapt your approach.