Your loved ones may have a mental illness, and you may not know it yet, but what happens when they are not well, is not exactly them. What might surprise you the most about it, would be that when they do start to be able to see how they were when they were not well, they can immediately see all the harm they have done.
This is another struggle with mental illness. What do you do with the pain you've caused when you were not well, now that you are well? A lot of the time what ends up happening, is that the person blames themselves for all the actions. This can cause someone to slip into a depression very quickly. It can cause many other problems as well, such as isolation from those we have had a bad interaction with during the time we were ill.
This can be a burden to carry that many people might not see when they talk to their brother, sister, father, son, or otherwise. This pain gets carried on with us as we go through life. It gets carried on the most when others aren't so forgiving.
It is important to the person you know with a mental illness, to let them know that you understand what they have been through, and that you know that it wasn't them necessarily that caused all the many problems.
When we have a long day, we don't always act like ourselves towards those we know and love. They know we are just tired, and they forgive us. This is very much the same kind of forgiveness that is necessary those with a mental health disorder, to get them back up on their feet.