Foam rolling yes. A standard foam one may not be enough depending on your pain tolerance though. I go into hips with PVC pipe or a lacrosse ball.
Don't get me wrong, you do need to eventually stretch and work to lengthen your hip flexors and basically "unlock" that joint, but if any stretch is aggravating right now, it's not the time. I know tissue quality is very hard to measure and can be somewhat subjective, but a basic gauge is the less pain you feel when you dig in with a lacrosse ball or your thumb on a troublesome spot, the better. You know you're making improvements when going over a troubled area no longer bothers you and just feels like a deep massage.
I'm posting a link here for some good stretches. thegriphouse.wordpress.com/2014/07/15/superior-hip-mobility-a-how-to-guide/
As far as training, how do single leg movements feel?