Welcome, yes here is a fine place to post, and this forum will not give you grief, it is a gentle place.
I guess you were plain unlucky, you have warm and loving nature, and an ability to talk out your troubles with someone you trust -and listen to theirs.
Those are wonderful qualities to have and I hope you experience with this other person will not change you too much. There are sadly some people who have the desire to 'play roles' with others and when they have had enough they move on, leaving a trail of distress behind them. They are skillful at 'pushing your buttons' to get the effect they want.
These is no real way you could know what was going on, it is bad enough in face to face situations where there are more clues. On-line it is just about impossible and even the wisest person can be fooled.
As for missing him, you said he was like a brother, so it is hardly surprising you miss that aspect of what happened. It had great meaning for you and it is so natural to feel great loss.
While being on social forums can be easy to get going on, and can fill one's day, it cannot really be a substitute being with actual people.
This place is an exception, it is moderated 24/7 by professionals and is safe as a result. There are no PM's here. It may well be others have had similar experiences on social forums, can I suggest you have a browse around and see what you can find?
I'd also like to ask if you have anyone in your everyday life, a family member or a friend perhaps you can talk with, who will listen and care. Trying to deal with this by yourself is very hard