I didn't sign up to a relationship with an RA.
^^^^^^^^^^^^^^^^
it appears you have answered your own question here.
Thinking your sadness has more to do with the compromise of your own standards, and partly his omissions, the fancy word for LIES.
This may not be what you want to hear, but be grateful it was only 6 short months you invested, and not 6 or 16 years. I am sorry, life does throw us curveballs at times.