I know it sounds easier said than done , but the fact that you don't know what to do is evident to Riley. He knows that your resolve is not 100% there.
So, get a whip , a long one like a dressage whip, or just a good long switch off a tree, and when he wont come forward, really reach around and swish it hard through air so it makes a big noise. Or, if that does'tn get his attention, really reach around and whale on him. He'll jump, be prepared, it will mess up your forward but at least he'll break out of the stubborn break.
It is true that you have to show him you're the boss, like that old saying. You might have to get angry. Just dont' be angry and helpless, be angry and forceful. Then , next time when you need to do this , you can do it without being angry, just being a leader.
Good luck. I bet things will take a turn for the better in time, and the weather will warm, days wil get longer. IT's all uphill from here on out!