I'll never forget the 4 hours I spent collimating my 8" dob for the first time. I almost gave up many times but eventually I got it and now I can collimate perfectly in less than 5 mins. Just keep at it is my advice, and if you still can't get it call your local astronomical society (even if you are not a member), I'm sure you could find someone there who would be willing to come and help you.
Good luck
,
Leon