Refrain sounds terrible

Hello, I´m new here and a beginner (male). 

I really struggle with the refrain.

For example Apologize by OneRepublic.

When I´m singing the verse it sounds nice. But when I´m singing the refrain, you can clearly hear that I couldn´t sing. It sounds not right. 

How can I fix this? Do you have any advise for me?

I´m a little bit desparing 

Hi, Franz! The chorus of that song in particular requires you to sing in head voice. See below for some head voice instruction and exercises!

Intro to vocal registers: https://www.30daysinger.com/tutorial/intro-to-vocal-registers/1

Head voice (beginner course): https://www.30daysinger.com/tutorial/30-day-beginner-course-with-camille/12