Well I'm not entirely sure what you mean by darkness. I think I used to feel this a lot when I was younger. I do, however, feel you on your problem deciphering how you feel and how to communicate it.

You should do a lot of thinking on your thoughts. When you feel something off but don't know what it could be you can make a list or diary entry of all the things going on in your head, write it all out. Do it each time you feel this darkness and then after you feel better read it and study it. Is there a pattern? Anything strange? Anything different from usual?

When you feel darkness try and pinpoint the emotion you are feeling and list anything that you think can cause it. After a while you will start to see patterns, things that trigger your emotions, maybe memories will come up illuminating where these feelings come from.

If you can find someone to voice ideas off of it is even better. I have made so many realizations talking to my SO about my feelings and can understand myself better.

As for relationships, it sounds so dumb but I warn people that I'm serious about getting to know better. I tell them some of my bad traits upfront and possible ways to deal with it. Obviously when I tel them I try to keep it light but I try to let them understand that I'm not a great communicator and am emotional and that sometimes I get confused. It has helped a lot when the darkness comes.

After having someone to confide in fully who does not judge me and gives me constructive feedback my dark periods have really decreased. I feel very balanced Nowadays which is a far cry from where I was years ago.