You've shared some interesting things about this guy you're living with - off hand, by what you have shared, he really doesn't appear to be mentally unstable - it appears that he just has some past issues he's dealing with.
You state: "I am living with a very nice person for 3 yrs" -
I am wondering how then can there be question about him being mentally unstable and especially that you have been with him for 3 years? Certainly, with one being mentally unstable, signs would have been there from the beginning.
You state: "He really doesnt believe in his mind that he does anything wrong."
How many people actually come forth and claim that they do anything wrong? I've been around for many years and have not witnessed too many people owning up to this fact. This doesn't mean one is mentally unstable. It just means that they are not aware that their actions or inactions cause disharmony for another....or that they are simply unaware of what is right or wrong. We seem to grow up differently with diverse ideals, beliefs, morals and many have not learned to respect others.
You state: "He will get mad at me for no apparent reason at all and leave."
Some people have a low tolerance level and are not verbal, they keep things hidden within...then, when they can't take anymore, they blow up - this is not so uncommon in people. This does not necessarily mean that one is mentally unstable - it can mean that they do not know how to express self properly! Not too many people understand why others do what they do, but that seems only natural, as we can only account for our own actions - and, not everyone is aware of that even....and not everyone knows what another is thinking, unless one is a "mind reader".....To you, you may feel he has no reason to leave - yet, it's possible that he does - and, therefore, that would not make him out to be mentally unstable. Again, we all have our own tolerance level and reasons.
You state: "Claims he does everything for me, yet doesnt realize the things I do for him."
Many people do not appreciate what others do for them, and many people do not voice their appreciation. Many people take things for granted, and many people believe that they are doing everything in their power - yet others do not recognize it....Could it be that perhaps he does feel he is doing for you - and perhaps he feels that you are not giving enough. Perhaps he fails to recognize and what you are saying is truth - however, whatever the case may be - this alone doesn't prove that he is mentally unstable.
You state:"Always paronoid that im going to leave him."
Well, this appears to be a form of insecurity - I know people who are paranoid that they will lose their job, or that they will be homeless, or that they will be without money or that something bad will happen to them because it seems they don't have good luck in life....but, does that make one mentally unstable? I don't think so - It shows fear! - Fear is not being mentally unstable -
You state: "His girlfriends have left him in the past."
This can be very hurtful for him. What did he do that caused them to leave? Was he mean to them? Were they mean to him? Perhaps it just wasn't meant to be that they remained in his life or vice-versa - certainly, that all led to the two of you being together, didn't it? However, if he was mean or abusive to them, then he may have a problem - but you have been with him for 3 years - would you be with him for that length of time knowing that he is mentally unstable?
You state: Doesnt seem to care about my feelings."
Oh dear, what can I say here! I believe many women have had thoughts/feelings like this - but, they say "Men are from Mars, Women are from Venus" - many men do not acknowledge there's a problem.....many men are not in touch with their own feelings, thus, certainly, are not aware of their mate's feelings. Perhaps one has lived a sheltered life, perhaps one is selfish, perhaps one is unenlightened, perhaps one is arrogant.....however, I don't believe his situation in this respect is a sign of being mentally unstable.
You state: "Is there a reason to worry?"
I would think you would be worried if he is physically, mentally, emotionally or spiritually abusive to you and/or if you are feeling scared to be around him.
You state: "Does he have some kind of depression problem?"
Perhaps he is depressed with the way his past life has been - even though he is with you. Many people carry issues from their past into their present relationship. Not everyone gets over it quickly. However, some people may show signs of depression, but that doesn't mean that one needs to be medicated for such or institutionalized for it. Depression can be like the weather - If it rains outside, many react to it...feeling all gloomy within....as opposed to a nice warm, summer day - sun is out, shining brightly...people react to it with cheerfulness. If it gets to the point that one can't function properly, then it's cause for concern, can be labeled depression, and one should seek professional assistance.
I saved your first question for last - "How to know when a person is mentally unstable."
Our society is dealing with many mentally unstable people, those that are living on the street, that have been casted aside, they just don't fit in with the status quo, when all they need is a warm bed to sleep in, a good meal in their stomach, a kind word, some love and compassion and/or professional help - that they have been so badly damaged, perhaps mentally, physically, emotionally and/or spiritually...that they are just not a part of the "norm"...that they are unable to function as another who can get up in the morning, brush teeth, get dressed, drink a cup of their favorite flavor of "starbuck's" caffee latte, go to work....etc. Many people who are down on their luck, due to unforeseen circumstances, are viewed by others as being mentally unstable. Perhaps one's past problems are so overwhelming that they just do not know how to cope, and that if financially sound, they would be able to seek professional help - however, with lack of proper counseling, they are broken.
Mentally unstable in my mind and eyes would be to the point of one physicially injuring another human being - and there's so much that you hear about on the news, I'm sure you understand what I mean. And, for one to truly abuse another, whether it be physically, mentally, emotionally, and spiritually -then there is true question.
In your situation, he is doing things that simply do not resonate with you - and you have a choice as to how much you wish to tolerate from him. If you have not gotten your message across to him - and/or he fails to acknowledge certain things in order to make you feel better about the relationship - then it's time to make a decision. Stay with him, accept him for who he is, hoping that maybe he will get the message and perhaps change for the better - or end the relationship.
Knowingly, if you truly believe that he is mentally unstable, you may wish to find professional help for him.
I hope this helps.
My very best to you.
Please take care.
Peace, Love & Happiness,
The Mystic Wave