Length of days is in her right hand - that is, wisdom, salvation, the knowledge of God and the things that he gives. It does not necessarily mean a long life literally; it means all the things for which a long life stands.
Do you have deep troubles because it is somehow your temperament? You often get plunged down into gloom and into depression. May be in your case you can't help it, may be it is part of your temperament. But then the devil comes along and he takes advantage of it and he tempts you to think that perhaps you are not a Christian at all. You were, but you have lost your way. You did seem to know something about these things but you have fallen and you have been somehow swept away. If you are a true believer, if you know the Lord, if your testimony is that you have sought him and you have found him and he has given you light and understanding then that serpent, the devil, may strike at you, but he cannot take you off this beautiful pathway.