Always falling apart back nine

All spring, I've been starting rounds well and playing the front nine 3–4 strokes better than my handicap, but as soon as I step onto the 10th tee, it's as if I've forgotten how to play golf.

If I've been hitting consistent fades, a hook suddenly appears. If I've been going around with a nice draw, the first slice of the round shows up out of nowhere. After that, I lose my tempo, start forcing shots, and overthink every swing.

If I'm putting for par, I don't make the putt. When I finally hit a proper drive again, I top the approach shot over the green. I've tried eating better and focusing on swinging smoothly, but nothing works.

Help?