First piece of advice: try not to stress too much. It won’t help, and looking stressed and anxious as you await/go through your entry interview might even make things worse by attracting more attention.
Second: prepare well. You may not always be able to use your smartphone during entry into the USA, so gather all your documents together and put them in a plastic folder to keep with you in your carry on luggage:
- Passport, printed copy of your ESTA and your flight booking, travel insurance (put the policy number and relevant contact numbers in your phone), AirBnB booking.
- Invitation from your girlfriend to stay at her home, which should include her address, contact number, and the dates of your planned stay.
- Recent/latest bank statement showing sufficient funds to cover the entire cost of the trip (make sure you have enough in case the plan to stay with your girlfriend doesn’t work out)
- Proof of ties to the UK that demonstrate you are a Carer for your father and that the arrangements you have made for cover are temporary during your absence in the US
- A prescription or doctor’s letter if you’re carrying any medication.
Third: remember that out of the millions who visit the US every year only a small % are denied entry and generally only the travellers who have had bad experiences on arrival at the border post about them. The main purpose of the entry interview at the border is to rule out any immigration intent by travellers who do not hold an immigrant visa. Typical questions in the entry interview include:
"Why are you visiting the USA?"
"How long are you staying?"
"Where will you be staying?"
"What's your first address in the States?"
"Who are you visiting?"
Answer briefly, accurately, and truthfully (for example “Tourism and visiting a friend”). Produce relevant documents only if asked for them.