adding text to anything in safari-- HELP!!!

I have been going nuts trying too figure out how to fix this issue: When I try to add text,usingng safari,in something like pinterest or Facebook, in the middle of the sentence it deletes it. It's beyond annoying and makes me want to throw my macbook across the room.

I have turned the double click on the mouse off...and even in the midst of typing this-- it keeps deleting text, moving the curser up to the middle of what I am typing and writing over it. What am I doing wrong?? how do I fix this??

HELLLLP!!!

 

Nope-- no insert button.
I have a Macpro- A1226. Any other suggestions-- would GREATLY appreciate it!!

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