[Grammar] I have to/must go to the doctor

[angelene001]

I've got a terrible earache. Painkillers don't help so I say:
1. I have to go to the doctor.
or
2. I must go to the doctor.

Which one is correct?
Is using the other one a mistake?

Same situation. I say to my friend:
3. Sorry, I have to cancel our meeting. I've got an earache and I'm going to the doctor.
or
4. Sorry, I must cancel our meeting. I've got an earache and I'm going to the doctor.

I'd be grateful for your help.

 

[Raymott]

I've got a terrible earache. Painkillers don't help so I say:
1. I have to go to the doctor.
or
2. I must go to the doctor.

Which one is correct? I'd say, "I need to go to the doctor."
Is using the other one a mistake? "Have to" is better than "must" (to me), but "need to" is best.

Same situation. I say to my friend:
3. Sorry, I have to cancel our meeting. I've got an earache and I'm going to the doctor.
or
4. Sorry, I must cancel our meeting. I've got an earache and I'm going to the doctor.

I'd be grateful for your help.

I'd say:
"Sorry, I'm going to have to cancel the meeting. I've got an earache and I need to go to the doctor." OR, "Sorry, I'll have to cancel the meeting."