Problem:
The user would like to display results of a view, grouped by a custom field.

Solution:
This use case is not supported out of the box by Toolset. Because the view loops over a list of Posts, and you are trying to loop over a list(Inspirations) of lists(Posts). But, this can be achieved with some custom PHP code.

Please check our article about a similar use case, grouping posts by year/month https://toolset.com/2013/10/how-to-group-views-results-by-year-and-month/

The trick is to order the view by the grouping element, in your case the inspiration custom field.

Relevant Documentation:
https://toolset.com/2013/10/how-to-group-views-results-by-year-and-month/