The e in werde is close to an Ä, pronounciationwise, like the e in error for example. Ü is a sound you cannot find in the English language. But to pronounce it you have to form an 'O' with your mouth whereas to pronounce an 'e' you make a broader/wider (?) expression with your mouth.
Never forget ("nie vergessen") implies time frame and permanence ("I will never forget" = "I will remember that forever") whereas not forgetting (nicht vergessen) could imply only remembering in that immediate instance and doesn't have the same context of this being a permanent memory.