Hey, Grammatically, you could change the words in the sentence as you did and it will still make sense. BUT, to 'get something off one's chest' and to 'get something or someone out of your head' are both idioms, so although people will still understand what you mean when you say 'get it out of my chest' or 'get her off my mind' it may sound wrong/weird. An idiom is a phrase or an expression that has a figurative, or sometimes literal, meaning. See more about the idioms here: https://idioms.thefreedictionary.com/get+off+my+chest https://idioms.thefreedictionary.com/get+something+out+of+your+mind

It is very informal language (and is actually grammatically incorrect), Done is this context means has.. so essentially, the man has lost his mind.

No real difference, they all mean the same thing.

The grammar is fine generally speaking but some sentences could read better, so: My aim is to create highly intelligent applications on websites, and also create websites that look beautiful, professional, are highly functional, load quickly and are Search Engine optimized. (I removed 'the website' from the first line because you are not referring to one particular website but websites in general). I have studied computer science from (early) childhood, and this has laid the foundation for the development of intelligent systems, which I now apply in various fields using information technology, such as e-commerce, marketing, finance and websites. (In this paragraph are you trying to say you have been studying computer science since you were little and that helped with your ability to develop intelligent system?, depending on what you're trying to say this might need to be tweaked a little). You can just say: While at university, rather than 'while I was at university'- what you wrote is not incorrect, as it what people would say but not necessarily what they would write. I constantly spend a lot of time studying new methods of developing web applications, and also increase my skills in e-commerce and marketing. This increased knowledge has enabled me to make the algorithms in the application faster and better, allowing me to do my job well.