Can replace WhatsApp Gmail

Whatsapp, new Google account, desperation

Hi all!

I have a problem and I hope someone will take the time to read through and help me. Maybe I'm just too stupid, but right now I'm completely overwhelmed and I can hopefully write everything down correctly.

I've had my cell phone and thus my Whatsapp and Gmail account since October 2012 and somehow I've never had to enter a password in the Playstore since then and I've never used the Gmail account. Now I don't know my password anymore and if I want to change it, Google says my contact email address does not match the account information and I cannot change the password.
Since my cell phone is pretty broken, I have now bought a new one, but I don't use it yet, because:
If I want to download Whatsapp to the new device, I need the account password, don't I? Or am I still somehow automatically logged into the new device via the SIM card? Or can I just create a new Gmail account and use it to download Whatsapp to the new mobile phone? Or is that not possible because my number remains the same and, so to speak, there is one account for each number?
I'm also afraid because I would have to remove the small one from my "normal" SIM card for the new cell phone and then no longer be able to put the SIM card back into the old cell phone if it doesn't work on the new one ?!

I am completely desperate and am already thinking about just getting myself a new number so that I can start from scratch. I am also super embarrassed about my ignorance and everyone thinks, I come from the Stone Age, but I would be sooo happy if someone would help me!

best regards
Julia