Can someone please clarify the different sentence structures I can use "下去" in?
have a look of the 國語辭典, "下去" has 4 meanings:

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000105631

1　自上而下 - from top to bottom

2　用在動詞後﹒強調由上向下之意 - used after a verb, emphasise the action from top to bottom

3　用在動詞後﹒表示事情繼續進行 - used after a verb, showing the continuous state of the action

4　用在形容詞後﹒表示程度漸漸增加 - used after adjective, indicate the degree is increasing

in the example, "接下去说" is equal to "接著说", #3

then, "你這樣下去會累垮的" is also #3