Firstly I'd say that if you're regularly passing out, even if it seems to be correlated to a particular type of working, then it's worth consulting a doctor just to make sure it's nothing else. For example it may be a dietary disorder causing you to run low on energy which prompts you to undertake past life workings and then subsequently pass out. It's probably nothing but something that's always worth being checked out.
That said I'd have to agree with the previous post. Past life workings are typically performed in at least an altered state if not a complete trance so that would be an idea to try. It may also be worth though spending some more time running through various meditation exercises to try and build up some energy and get more used to general workings.