I think it is OK to trust people at face value IF they have already earned that trust. For example, if you have a friend that is always truthful and who never exaggerates, then it is OK to believe him (unless you have a reason to think he is lying). However, you should also understand that if it's someone you don't know very well yet, then they haven't earned your trust. So you should try to keep an open mind, not necessarily think they're always lying, but not necessarily thinking they are always telling the truth either.

You are not slow, you don't have anything wrong with you. You just need to remember that trust is an earned thing, that happens over time as you get to know how truthful someone is. If someone is not always truthful, they may never earn your trust. That is OK - you do not have to trust everyone even if you have known them a long time