Can't log in at all! "Warning Cannot modify header information.."

I’m a total WordPress dunce I don’t know anything, my site was made for me and I’ve just been using it to do my blog.

I was trying to get Google analytics to work and pasted the code into the main template, this wasn’t working so I pasted it into every page until I suddenly couldn’t log in and was faced with this

https://fashionscholar.com/wp-admin/

I’m only using this forum now with a new account as I can’t get into my actual account because of all this! I haven’t lost it forever have I?

If anyone can help I’d be so greatful, I dont earn much but will definitely be willing to pay!!

Thanks in advance