'Only now' is slightly formal/poetic-sounding, mainly because it needs an inversion of the verb and subject (as with a question form). 'Only now do I realise how foolish I've been.' So your sentences above would be: Only now does it dawn on me. Only now do I know the truth. The inversion can also produce some slightly awkward-sounding expressions, so it's probably best avoided unless you have to use it!