I'm at the end of my rope with potty training and I don't know what to do anymore

I have a little boy who turned 3 last August. He pooped on the potty once and we praised him for it and then he regressed and wouldn't even try. So we told him we would have a "poop party" if he went poop on the potty. He finally did that in January and we kept our word. Pizza, movie, and a power rangers toy. We bought him underwear and he was happy.Aaaaaand after a week he regressed again. He absolutely refuses to try. And I have identified when he needs to go. He has hiding spots in our house that he runs to and poop in his pull up. When I see him go to these areas to "hide" we tell him that he needs to poop on the potty. "But daaad I don't need to poop."So I'll take him by the hand and I essentially make him use the potty. But he won't go. He has sat there as long as 30-45 minutes with books and activities and will whine the whole time.Then once I've decided to let him off and he puts on his pull up he will come to me after 5 minutes with "I pooped my pants."So we clean it up and I tell him it's not fair to make a mess that someone else has to clean.It's gotten to the point where he is constipated and now is afraid to go because he will hold it in for 3 days and then it hurts because he's passing out a Buick.Our doctor prescribed a half capful of miralax and it works. His stools are loose, but he just will not even try.I'm terrified because we signed him up for 4K in the fall, but if he's not potty trained they won't let him go. I'm sick of the pull ups and I'm absolutely anxious and frustrated and depressed that he won't even try.Right now he's bawling in the bathroom because I won't let him off to go hide in a closet and poop in there.And I'm super anxious that now he's just going to hide it from me and I'm going to see hidden piles because he thinks it will be easier.I don't know what to do. I'm literally 5 minutes of tears away from a full blown panic attack because this has been 6 months of struggling...edit: I let him off because I'm struggling with depression and anxiety right now and I'm about to lose my temper and I'm not in a good place right now. via /r/Parenting http://ift.tt/2kVZem5