Depression

I am sorry to hear about your situation. It is normal to re-evaluate how a marriage is going after 7 or 8 years and see if it is functioning well.

Life is (somewhat) about choices. If the marriage isn't working for you, Islamically speaking, the first step is to try to make it work (for instance, as mentioned, through communication or a marriage counselor, if he is willing). That said, it requires the interest and commitment of both people in a marriage for things to change.

If he isn't interested in changing, then you need to decide what you want for your own life and future, and whether to accept the situation as it is or to try to move on  (obviously, taking into account all factors, such as how the relationship is otherwise, financial matters, whether you have children and what you feel would be best for them, etc). While divorce is discouraged in Islam, and, statistically speaking, women tend to suffer more than men (financially and emotionally) after divorce, it is also not good to harm yourself or stunt your growth and potential if there is no greater good behind it.

This is ultimately a decision that you would have to make for yourself since no one is in your shoes and can fully understand your situation, especially if depression is a factor. 

I would suggest in any case - and I hope I am not overstepping my boundaries - that regardless of whether or not separation might be in the future, it is always healthy to have friends and associates who can be a safety net in a time of crisis. This is true both for yourself as an individual, but also for the family, as we never know what will happen - what if he were to suddenly be in a coma or something? If there is any way to make friendships, even online, it would be helpful not only psychologically but also on a practical level. 

(Indeed, in the current world situation, many of us are discovering the value of having a safety net.)

I would also point out as tactfully as possible that, oftentimes, when someone is extremely suspicious and untrusting, it is because they have things to hide, or else they have behaved questionably in the past. Otherwise, normal people are not usually extremely suspicious or untrusting. I am just putting that out there, and that may not at all be the case in your situation. It is just an observation about human psychology. 

Life sometimes doesn't have easy answers but prayer for guidance is also always a good start. 

I am sorry to hear about your difficulties (or the difficulties of the person you are asking on behalf of).

To add to the below response, I find that marriages tend to work out best when the husband and wife feel they can talk openly to each other about their lives without feeling they have to keep secrets. It can be difficult to build a deep relationship when there are big parts of one's life one feels that one can't discuss.

At the same time, real life being what it is, sometimes it doesn't work out to share some things and sometimes one person will use them against the other if they are not entirely of good character. I can also understand not wanting to open up about something personal or sensitive to the whole family and having them weigh in on it or talk about it with each other.

Anyway, there is no shame (or at least there should be no shame) in mental health conditions, just as, indeed, there is often no shame in the other things that people, often women, feel compelled to keep secret for social reasons. 

I do agree however that when a person finds out something later, oftentimes the reaction is worse because they feel deceived and that it is a betrayal of trust.

But you have to make whatever decision is best - perhaps consider doing istikhara about sharing it, if you are genuinely unsure?