In your screenshot, right-click "Kopfhörer"->Eigenschaften, then tab "Pegel" and you will see a meter for "Mikrofon". This is a Windows function that gives you immediate feedback to your Kopfhörer what the Mikrofon records. If you use this, you don't need to use the monitoring feature of OBS.
On the other hand, it is not necessary to have this kind of feedback.
Much more useful is if you don't monitor at all and make a test recording instead and listen to that recording how you sound. Speak loudly, speak silently, play music, hit the table with your hand, hit the mic with your hand, click violently with your mouse and type keys violently and check how this all is being heard in your recording. And listen to your completed streams. This way you see what noises you should avoid, and how loudly or silently you should speak. While being on your stream, you usually don't have the chance to constantly monitor your voice. If you stream alone, you are actor and director at the same time, so it's difficult to act and and verify the acting at the same time.
