I get the appeal — that line is a classic little kick-in-the-pants. From my own little experiments making gifts for friends and showing mockups to people over coffee, the emotional pull here is strong: people love motivational lines they can read first thing in the morning. That said, the exact phrase you typed, 'it's always seems impossible until it's done', has a grammar hiccup that will make some buyers pause. The smoother, better-known version is 'it always seems impossible until it's done.' Fixing that will widen your audience without losing the sentiment.
Design matters more than most folks admit. On a mug you want clear legibility, contrast, and a focal point. Try a bold short type for 'impossible' and softer script for the rest, or strikethrough 'impossible' and replace it with 'possible' as a playful reveal. Colors that match kitchen vibes — warm neutrals, deep navy, or a pop pastel — tend to sell. Consider printing method too: sublimation on ceramic for dishwasher-safe durability, or enamel for outdoor campers if you want a different market.
If you want to test it, do a small run with two variants: the corrected phrase in a minimalist layout, and a playful illustrated version. List them on a marketplace or show them to coworkers and take notes. Pricing around the mid-range for quality mugs tends to balance perceived value and impulse buys. Personally, I’d go with the corrected line and a simple, tasteful layout — it’s the sort of thing I’d grab for a friend heading into something big.