You can always say 'I thought you had quit', but native speakers usually say 'I thought you'd quit'. G Warner explains that it more formal and might be considered confrontational.
As to whether 'I thought you'd [you had] quit' might be interpreted as 'I though you'd [you would] quit', the clue in this example is the previous conversation with your friend. You wrote 'Someone has told me she has stopped smoking'. Using the usual rules of indirect quotation, this means she actually said 'I have stopped smoking', not 'I will stop smoking', otherwise you would have said 'Someone has told me that she would stop smoking'. So this time when you meet, both of you have a (general) memory of that conversation.
That relies on knowing the previous conversation. If you simply say to me, 'I saw Betty smoking yesterday. I thought she'd quit', I don't know the previous conversation, and might just be caught between 'I thought she had quit' or 'I thought she would quit'. But as a native speaker of English, I will very quickly think that 'I thought she had quit' is a more likely interpretation than 'I thought she would quit'.
Conversation almost always has a context - who the people are, where they are talking, when they are talking and what has already been said or happened, either in this conversation or a previous one. We rely on clues from the context - in this case, the previous conversation.
Note that this issue only arises with a small number of verbs: the ones like put-put-put, hit-hit-hit or in this case quit-quit-quit, or come-came-come, become-became-become or run-ran-run. If you had stuck with 'stop', then your choice would have been 'I thought you'd stopped [< you had V-pp]' or 'I thought you'd stop [
One day in class I said to a student 'I wish you'd come earlier', which raises the same issue.
Best Answer
Opportunity is a more formal equivalent to chance.
Much of an opportunity is more formal than much opportunity.
You can easily fix spelling mistakes such as "councelor" if you make sure your browser is checking your spelling as you type.
"Please allow me to elaborate on my learning experiences" sounds fine.
However, I don't think you really need to explain why you are giving the guidance counselor more information about you. When a guidance counselor is responsible for several hundred students, it really isn't possible to get to know them all personally.
I hope you will make an appointment to meet with the guidance counselor in person. Best of luck with your applications