Just keep practising! Duolingo works by repeating words and phraes in a structured way, so your memory of particular phrases will improve as you practise a little each day.
Does anyone know any good phone keyboard apps that have Welsh as an option? Edit: I've been using SwiftKey for a few months. It has Welsh autocorrect. It's pretty good, but it works better on Android.
Windows - 'To Bach v2'
Android - SwiftKey?
MacOS and iOS - not needed. Use Welsh keyboard layout in MacOS. On iOS, set 'Region' to UK then just keep keys pressed to bring up pop-up accented characters including ŵ, ŷ.