As others have said "tuff" is an alternate slang spelling of "tough". In my personal opinion the spelling "tuff" feels uncool and outdated. You can look at the ngram and draw your own conclusion.
Here are some examples where tough and difficult have similar meanings:
I'm having a tough/difficult time fitting in at my new workplace.
My father just passed away and I've been having a tough/difficult time lately.
If we compare
fairly/quite bad. I would say that the former implies some sort of difficulties in performing a task whereas the latter just implies a generally bad situation.
I've been trying to set up a new webpage but the web server is using a outdated OS, the electricity went out yesterday and then I forgot the admin password this morning. I'm having a pretty tough/difficult time with this.
A bird pooped on my windshield, I forgot my lunch at home and my boss told me that I will have to come in to work on Saturday. I'm having a fairly/quite/pretty bad day.