You likely need a new HD cable! Sadly the original cables only supported the slower SATA II (3.0Gb/s) HDD Apple used originally. They later upgraded the cable a few times! The last version is able to support SATA III (6.0 Gbps) HDD's or SSD's. So I would start there MacBook Pro 13" Unibody (Early 2011-Late 2011) Hard Drive Cable and here's the guide MacBook Pro 13" Unibody Early 2011 Hard Drive Cable Replacement
Here's a bit more on the cable issue Your Hard Drive Cable Is A Ticking Time Bomb I only use one strip as that's all thats needed to hold the cable off the rough uppercase.
Even still there is still one more issue! Often I find people kill the new cable by damaging it! You can't crease the cable as that damages the thin foil wires within it. I use an old BIC pen ink straw (empty) to help me form a curl in the cable I don't want the radius of the cabal to exceed the diameter of the straw when the cable needs to bend around. And don't fold the cable at the connector either.
Lastly, I would recommend after you make the repairs, make a full backup, then format the drive and re-install the OS. If you haven't I would also start off making a bootable OS installer thumb drive (32GB) following this guide to speed things up How to create a bootable installer for macOS then restore your stuff as its likely you have a few corrupt files.
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