Thank you Trippin, thank you riotgrrrl!
Puuh, at this point I feel totally numb and helpless. Your opinions sound logical and I seriously put trust in your words. Now I have to think about how to move on, what to do next.. And somehow I feel like retelling the whole story again because it's so unbelievable that you do not blame me.. I'm like "Did I make them understand correctly? Did they really get the whole thing?" Thank you so much for your support!
I know I love him, and I also know that love only cannot be enough to make things work. I want to be with him, and I don't want to give up on him.
Yesterday and today, everything between him and me went fine, even though I noticed those slight mood changes. He can be laughing and happy, making jokes and so on, then some minutes of silence follow, and eventually it can be that he's getting kind of distant/ reserved or even a bit rude (when giving answers). Then he somehow transforms into something like a "time bomb" - The moments I am so afraid of.
It happened a few times that he told me how much he loves me, holding me tightly and so on, and then he suddenly let go of me and got very frustrated and bad-tempered. Once he justified that behaviour by referring to a "reminder" - so there was something popping up his head that reminded him of how bad I am/ of my lies. But it's so strange, and quite often I've got the feeling that he's got those two faces.

I remember once we had a party, about 10 friends were over, we were singing karaoke, and my boyfriend wrestled with a very good friend of his, just for fun, since he's kind of a professional in these fighting-things. And for some reason at some point his mood changed from "super happy" to "super angry". I think the actual reason was that his friend held him somehow incorrectly when wrestling, but that friend apologized maybe ten times as he hadn't done that on purpose. That friend felt horribly bad and said "sorry" again and again, looking my boyfriend in the eye. But there was no chance to help the situation. My boyfriend threw everyone out of his apartment except me, listened to the music at high volume, threw a bottle against the wall, and after he had cut his arm he finally fell asleep downstairs on the sofa. The next morning I left when he was still asleep, and in the afternoon he sent a message like "I'm sorry, I had a crappy day at work plus too much alcohol". I think that was the only time he didn't blame me and my lies for his extreme behaviour.

Why is he like that? How can his mood change so drastically so that I even feel/think he's got two faces? I know he was depressed in his past, and started hurting himself in his late teenage-years. The last 3 years he didn't hurt himself at all, but started doing it again because of me.. He says he's depressed because of me. But isn't this far beyond depression?