Don't do anything that might damage your teeth without first talking to a real dentist
. I absolutely would not use the online kit. And I would not wear the old one if it hurts.
Second, if you are in the US (or I suspect any modern Western country) this is between you and the dentist. If you are over 18 your dentist can not tell your parents anything about your treatment
. It is illegal for him to discuss your medical history with anyone (including your parents) without your consent. You are an adult (I am assuming you are over 18) and you are responsible for your own health care. Your dentist now works for you. If he or she is yelling at you, you have the right to just leave and get a new dentist-- although most dentists are professionals and won't do that.
If I were you, I would go and talk to your dentist. Tell the truth about what happened (it isn't that uncommon for people to lose retainers at college or to go a year without wearing one). It isn't the dentists job to be upset with you and it is unlikely he will yell at you. The dentist's only job is to recommend a treatment and to explain the various options to you.
Ask the dentist what your options are. Choose one of the options. It is possible you will get a new retainer. It is also possible that at this point it isn't worth doing anything since if your teeth haven't moved by this point, they aren't going to. But ask the dentist for her advice.
You don't need to say anything to your parents.