What you should do is try to get access to your old account by security questions or even getting in contact with yahoo via email or calling them to resolve the problem. Then there should be something called 2 step verification on yahoo if not make a password at least 10 characters long and at least 2 numbers it may be ling but it is harder for hackers to decode the shorter the password the easier. Anyway 2 step verifacation is where you need to enter your password and another form of acces like a phone call from yahoo with a 6 digit code or a security key to access the account it is pretty much a second password but not one you have to remember because it is differnt every time and it is the best way to keep your account secure on top of the long password it also helps to do monthly security checks and if something is wrong or doesnt seem right report to yahoo and they should take care of it also change or update all information like phone numbers, security questions, ones nobody really knows.