The 46-year-old actor split from Westfeldt nearly two years ago and said being on his own after all that time is not enjoyable reported InStyle magazine.
"It's hard. It's hard to be single after being together for a long time. It's really hard. It sucks," Hamm said.
The "Mad Men" star also called himself a narcissist but insisted he is a good partner.
"I may be a narcissist in the way that most actors are, but I think it's not to the detriment of those in my life. I try to be a good friend, a good partner, and all that stuff.