It is a sad reflection on some fish stores that this is a very common occurrence. Please don't despair. Success is within your reach.
Before you go further, plan what you want in the end, and post your questions and ideas as they arise, many on here will have suggestions and advice to prevent any recurrence. To think how many fish I lost in my first days (decades ago now) when I did the same, and there were no such forums (not the internet or computers...that's dated me
) to help.
Best of luck, and stay with it. We're here.