custom field formula: convert text field to date and output month

How to convert a text field to date format?  I have the text field textfield_date__c in the format "dd.mm.yyyy"   I tried the following formula in a custom field to get the month:  

IF(
NOT(
ISBLANK(
textfield_date__c
)
), 
MONTH ( DATEVALUE(textfield_date__c) ),
null
)

This generates an error in the field output. Do you have any idea how to solve the problem?