Ultramarina, I had the same problem when I started freelancing and a former co-worker who knew my abilities told me to estimate and bill as a lump instead, that helps get around the discussion of how long it takes or doesn't take. Once clients know you, they understand the higher hourly rate is worth it rather than paying someone less efficient half the price for a quarter of the results.
Unfortunately for me, estimating and selling my services are not my strengths.