Ah! A VERY difficult area for Russians! You don't have articles. I have a lot of Russian students and they all say the same thing - that it's very hard to understand where to use them and which one to choose. Grammar books help, but they also cause confusion... because there are so many 'rules' and so many exceptions to the rules. What Russell has said in the comments section is a good start. A lot of it is in the speaker's head... have I mentioned the doctor before? Does the listener know which doctor I'm talking about? Does it matter if the listener thinks I'm going to my usual doctor or just any doctor? The beauty of articles is that you can be very precise, and you can provide a lot of information to the listener by using a particular article. This is not an answer to the question really. It is more a suggestion... do not worry! Keep practising, do lots of worksheets on articles, make a note when you are reading about the situations where the writer uses an article. (You could look at my last paragraph! THE/A/AN)

As Jmat says, most people are usually registered with the same doctor, so when we say 'I'll go to the doctor' (or the 'doctor's') it means 'the doctor [that I usually go to]'. It's the same with dentist. You go to 'the dentist' every six months - and it's understood that this is your regular dentist. However, if you suddenly had toothache on holiday in another town, you might try to find 'a dentist' ie any dentist. It's the same when we say sentences such as 'He's at the pub' 'She goes to the gym' 'I went to the supermarket' 'He ran round the park' The suggestion in all of these sentences is that there is only one pub, gym, supermarket or park that the person usually goes to. 'A gym' would be a random, unspecified gym, whereas 'the gym' means your usual gym, or the gym which you belong to.

You can say "I'll go to a doctor" just as easily as "I'll go to a psychiatrist." Likewise, you could say "I'll go to the doctor" and "I'll go to the psychiatrist" if the doctor/psychiatrist is known to the speaker. The thing is, most people go to the same doctor consistently, so they've gotten into happen of calling their doctor "the doctor', even if the listener doesn't know who their doctor is. People say this to the point that people just think of "the doctor" as a phrase and don't think anything of it. People rarely go to psychologists, and they definitely wouldn't want to imply that they see one consistently (if at all). If someone does see a psychologist consistently and they don't feel like they need to hide it I don't see why they couldn't say "I'll go to the psychologist."

A definite article is SPECIFIC. That is why you use the word, THE, because it implies something that is known to the speaker/user in his or her mind. An indefinite article is UNSPECIFIC. When you say "I'll consult A doctor", you are not showing that you have a doctor in mind. Just a doctor, which could be one in the hundreds in your region. I took this off grammar-monster.com/ , i hope this helps to make it clearer. I have found a solution to the problem. (The solution is not yet known by the listener. The problem is known to the speaker and the listener.) I have found a solution to a problem. (This would attract a response like Good for you. What problem? The speaker's use of a tells us that the problem is not known to the listener.)