Tightening the girth won't do a thing when the actual saddle is too wide, which is why the gullet is so low on the withers and like was said before, it isn't balanced. There may be a way to build it up using a pad to make it fit "alright" however it'll never be perfect. I had the opposite issue with my amazing pink seated barrel saddle, debated getting a new horse instead of selling it! (just kidding, kinda haha). It was much to narrow and instead of sinking like yours, mine pinched like crazy. Saddles just have to go and be so difficult.
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