can't log in to app at all

been trying to log in to app, to no avail. it says ive put the wrong username and password when I know they are right use that login in for this site and devices share login info, even when I type manually. please help its been happening for a while but at first I thought I had the login wrong but ive checked it heaps and still the same. thanks in advance

What app are you exactly talking about? Depending on the security features of that site, you may really have trouble logging in. In my case, I had an experience where I failed to log in because I misremembered my password and forgot that I change a small part of it because of the password requirements of the site. If you have a recovery email or other account, then that would be helpful. Otherwise, you need to contact the site to see what you can do.