It really does depend on the act being considered. For something to lower culpability, it must significantly affect the strength of the will in choosing the act. Severe depression can certainly weaken a choice of the will enough to lower culpability, making a sin of grave matter a venial sin instead of mortal. The sins that you mentioned would seem to be of a nature that would fall under this category. More severe sins such as murder, abuse, etc. take a larger movement of the will to choose, so I doubt depression would lower one’s culpability for committing them (though this is a generalization - judgments of culpability are always specific judgments).
What Swan said was excellent. If you suffer from depression, get help for the condition and try as hard as you can not to let it draw you into sin. If you are doing that, and going to confession regularly as we all need to do, you are on the right path.
As for suicide, it is my understanding that most people who have bipolar disorder and commit suicide do so in the manic phase of the disorder rather than the depression phase (or are at least more likely to go through with it). In the manic phase, good judgment can be seriously impaired. Such a person would probably (depending on particular circumstances again) have very little culpability. Swan and Christy Beth may be able to speak to this better than I, but I think people in the depression phase probably have more self-control than they do in the manic phase. However, even in the depression phase - as Christy Beth shared with us - your judgment is seriously impaired by the distorted perception of reality. When everything seems hopeless, how clearly are you choosing when you attempt to end it all? Certainly psychological disorder is always a mitigating factor to one degree or another in cases of suicide.
Christy Beth - I hope you are doing better now!
