You are always doing the best you can, based on your current belief system. Life has designed us with a built-in mechanism for survival. To repel from pain, and move towards feeling good. It's in all its creatures, big and small.
I think what trips us up is that it doesn't always APPEAR that way. If you hear someone saying “you mean the person that slits their wrist is doing the best they can?” the answer is “Yes”. At that point in time, ending their life appeared to be the best way to end their pain. If they knew a better way, they would have taken it. It's only logical.
At any moment each person is always doing the VERY BEST he can, based on his total conscious and non conscious prevailing awareness and which is within his capabilities, energy, time, and developed talents and abilities.
How many times have we heard, “if I had it to do over again, I wouldn't have acted any differently, based on what I knew at the time”? Precisely. Based on what you knew at the time.
There are reasons for the things we do. Most people have not investigated what those reasons are, but that doesn't mean they're not there. If you were to trace your thoughts from behavior back to belief, you would see a logical progression of reasons for each step you take.
Think of a person like a huge mainframe. You see the nice software interface, but be assured that there are millions of calculations taking place behind that pretty window. They're not necessarily subconscious, you can see them just like you can see code. The only way I've discovered to get to that code, is to identify those underlying ideas and beliefs.