How can I add {% if %} for a field name?
You copied field.html.twig to a new file, field--field-additional-text.html.twig. After clearing the cache, Drupal will only use this new file for that field only. Now you do not need logic in either template.

So you now can make changes, for example:

{% for item in items %}
  &lt;div class="additionalbreaker"&gt;{{ item.content }}&lt;/div&gt;
{% endfor %}


^ This means, for every item in this field (single or multiple values), print each value wrapped in this div, with this class. However, within a field template, you cannot output different fields as one, the sole purpose of a field template is to render the current field.

Read up on theming Drupal fields for more information.