You might be a bit hard on yourself Nick. I did have a look at what you were talking about and that thumb over the 4th fret is not easy. I would just keep at it, but not to the point where you are making yourself miserable. I often try things, stop for a while, go back to it. Over time, things usually come around.

Have a look at what I am doing, maybe it will give you some ideas. I find on really tough chords like this I need to experiment with different angles and positions for my elbow and even fingers.

https://www.youtube.com/watch?v=esHJNcyi__A&feature=youtu.be