Thoughts, opinions, advice wanted for my stupidity.

I recently purchased a Trek Madone 5.2 made back in like 2011. In good condition, but the gears weren't shifting well, so I tried to make it right. I ended up finding that I couldn't get the last gear, even though I tweaked everything, it wouldn't shift to the last cog. I ended up destroying the shifting cable cuz I'm stupid. It was out of frustration.

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That being said, while it's currently being worked on, why couldn't I adjust it so the chain could fall into the last cog? I maxed out everything. There was nothing else I could do other than pull the cable out more... But I'm not well versed in this. My last bike I had no problems while doing this, but this time it just didn't work

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Any advice, thoughts, or general information regarding this would be appreciated