I've been thinking about this for a while. This is my 2c fwiw. There are a few barriers here.
Firstly, any attempt at describing a feeling and the reasons for it always falls short of the truth. There's always more behind it. For example, the current feeling of anger I have towards someone is really just pent up anger thats been sitting there forever waiting for an outlet that this person happened to provide. I could point to him causing me to be angry, but it's not the whole truth. If I didn't have the pent up anger there waiting for an outlet, I could have responded without anger.
Secondly, severe long term depression is a concoction of so many suppressed and repressed emotions that it's reasons are largely unconscious. Not only are they more layered than the immediate causes I'm thinking of, but most of the emotions have been repressed so long that half of those layers are no longer even accessible consciously. I understand that I'm feeling a certain way, and these are the options to deal with it, but why I'm feeling that way is always at least partly a mystery.
Thirdly, while an audience such as this may still be able to comprehend to some degree what you're going through because they have similar emotions / experiences to draw on, the majority of people who have never experienced severe long term depression will never really understand. My experience in this regard comes from being on the other side of this conversation when a long term friend revealed he had attempted suicide and tried to explain why over many hours. Bear in mind this is someone I'd known and been close to my entire life. It still wasn't possible for me to comprehend or really even empathise with him, because I'd never felt depression to that extent. I find I can empathise a little with someone whose decision was triggered by severe money troubles, or health/pain/disability, or even terrible relationships / losing loved ones, because I fear those things myself, but for someone with none of those triggers talking about mental health/depression/anxiety, it was like there was an impenetrable wall between us. I couldn't understand, all I could do is love him and support his decision.
Then on top of all of that you have the fears behind telling other people, which again lead to unconscious repression of emotions and their reasons in order to reinforce why it's pointless to even try and thus avoid facing the fear. Unfortunately that fear is blocking the strong desire to tell someone, which I think is why this predicament is so common.
I doubt that's any help. They're not really solutions - more attempts to explain your predicament. Although I think you can be sure virtually everyone is in the same boat!
