In the beginning of the game I missed the missing amulet quest. I sent the questgiver to train without asking about it.
Now I can't talk about it anymore! The amulet is needed to finish the game. What can I do?

You're playing with Eric right?

I think that's the one you get right in the beginning in the old camp. So if you've already lost your amulet (talked to that guy once), this means you have to ask around Berezhki village (It's 2 maps to the left from the old camp) and some townsfolk will point towards witchdoctor in the cave there. Hopefully you will continue this quest further yourself, otherwise i may try to help.

Unfortunately I can't find anyone who talks about it. I guess I have to start over...

Did you find witchdoctor in the cave of berezhki? No point starting over. There's only 1 bugged quest in the whole game as far as I can tell and it's not this one.