To resolve the issue of being unable to access your MacBook (2015) due to a forgotten password, the most effective solution is resetting your password using your Apple ID. Restart your Mac, and while it is booting up, hold down Command + R to enter Recovery Mode. Once in Recovery Mode, select "Utilities" from the menu bar, then choose "Terminal." In the Terminal window, type resetpassword (all one word) and press Enter. This will open the Reset Password utility. Follow the prompts to reset your password using your Apple ID. After resetting your password, you will be able to log in and manage your applications as needed.