How easily could someone break into a password protected text file like Word?
For modern versions of Word (Word 2007 and later), it appears that Microsoft has got the encryption pretty much right: They use multiple iterations of SHA-1 (since Office 2013, this has been replaced with SHA-512) for key derivation, and they use AES with a 128-bit key for encryption, compared to the older schemes that used everything from a 16-bit key to a 40-bit key. So if you are using Word 2007 or later, and ideally Word 2013 or later, it should be reasonably secure. Formats compatible with versions prior to 2007 will not be secure at all.
However, at least historically, Word has been known to litter temporary files all around itself while you are working on a document. I'm not sure if this is still the case, but remember that Word is a word processor; it's not really designed to keep secrets against a determined adversary who has access to your system. The document password protection only really helps if the adversary can get their hands only on the intentionally saved document file, and nothing else. There are many threat models where this protection would not be sufficient, and many of those would seem to apply to lists of passwords.
So it stands to reason that you could get reasonable protection against certain threats by keeping your passwords in an encrypted document maintained in a recent version of Word making sure to use non-backward-compatible formats.
On the other hand, by using a tool specifically designed to securely keep a list of passwords, you get one that is much smaller (thus far less risk of a bug having crept in, and far more likely for a compromising bug to be taken seriously), tailored for the purpose (thus far less likely to litter plaintext all around), and intended for the purpose (thus likely has useful features such as the ability to generate passwords, set password expiration dates, etc.). That's a password manager.
I discuss this also on my personal web site, where my recommendation is to use a password manager. (Actually, that's currently my second top advice, second only to do not share your passwords with anyone.)
Whichever way you go, wherever you store your list of passwords will require a high-grade passphrase. Remember that anyone who is able to guess that password will have access to all of your accounts; treat it accordingly. I suggest reasonably long Diceware passphrases (also) for this.
Some programs are made for encryption. Other programs are not.
Microsoft Office products' password-protected features (such as preventing certain spreadsheet cells from being edited, or a document from being viewed) are historically meant as convenience features. In 2003, the average office worker wouldn't open documents with special software to get around limitations. These days, such knowledge has matured and tools have been made widely available.
I don't know what the current state of affairs is in Microsoft Office Word, but I'd generally recommend to use special purpose software, i.e. a password manager. They are made to be secure and have features such as automatically erasing keys from memory.
To generally answer your question though: it could be secure, provided that the software you use does proper encryption. Encrypting a document with WinRAR would be secure, for example. But it's just a lot easier to use software that was made for this purpose, like KeePass.
For storing the password you are using password protected file and it is not safe. You should try https://www.truekey.com/ for storing the password as it is the product of Intel. In this you just need to remember the master password only and your other password should be managed by TrueKey itself.