Add Next and Prev button

Hi there,
i’m trying to get a next and a prev button in addition to the dropdown. So when someone click the Next button the next post should be displayed in the order of the dropdown and the same thing with the prev button.

I added the button like this.

$select .= '<input type="submit" id="fieldNext" name="next" value="Next">';

And now i can go through the posts but they do not change i see that i can switch them in the dropdown but the content is not displayed.

Can you help me at this point please.