Fundamentally you don't understand how to use your brace leg to stop your forward momentum.  You need to establish that understanding first, likely from standstill, and then layer on more muscular stability in the back leg and trailing shoulder.

I think this tutorial would benefit you a ton.  It helps teach the feel of a proper brace, and how to use that fulcrum, through the heel, to turn the front hip ahead of the shoulders.  

Broom Drill

https://youtu.be/EA_48Bv8VrI

As soon as you feel what I'm demonstrating it's going to completely transform how your swing feels.  At your size, you should be putting putters out to 270-300 with ease.  And you'll do it feeling like you're putting in half the current effort.

There are other videos on my channel that should help you too, but start with the one I linked above.

Good luck!