Coming from a person with a diagnosed mental illness: it's okay to be self-diagnosed.
No seriously, it is.
There is a lot of romanticization of mental illness, and click-bait articles/Facebook listicles have made it easier than ever to get a list of "symptoms" and to diagnose hundreds of people with, usually, a form of anxiety or depression. No one is denying this.
However, self-diagnosis is a key element in actually getting help from a professional, especially if you're an adult. I was lucky I was diagnosed as a kid when people noticed I was having more severe mood swings than a seventh grader should be having, and I was having suicidal tendencies. That's usually not something mentally healthy people do. What if no one is there to guide you to a therapist or counselor or any kind of doctor, though? You know yourself best, and the beginning of receiving professional help starts with self-diagnosing. The path to therapy or a doctor starts with thoughts like "Well, I haven't showered in a week, I have no motivation, death is pretty fascinating, and I have absolutely no interest in any of my hobbies or friends anymore. I think I might be depressed. I think I might need help."
Self-diagnosis versus a professional diagnosis also bears the concept of a professional diagnosis not being accurate, or the professional pushing the person into medications and therapies the said person wouldn't be comfortable doing. The legitimacy of professional diagnoses is questionable, to say the least when anti-depressant, anxiety, and psychoses medications are sponsoring and advertising in doctor's offices. Different mental disorders might become popular too and cause massive over-diagnoses, like multiple personality disorder (now referred to as dissociative identity disorder) or ADHD. Can we really say one diagnosis is truly superior than the other with these influences in the professional world? This isn't to invalidate people with professional diagnoses, but rather to look critically at what's really going in the world of professional psychology. One must also consider the possibility of someone not being able to afford a professional diagnosis, as visits can be rather pricey along with medication and therapy.
If you're seriously concerned that you do have a mental illness, and for whatever reason, you don't wish to go to a professional, here's some ways to help with that diagnosis:
1. Visit multiple (credible) websites
Personally, I wouldn't trust any social media sites like Facebook, Tumblr, or Twitter for symptoms/validation. Try medical websites and websites that might advocate or fundraise for the mental illness; most will be the first sites listed when you search the mental illness. Those will have often a list of symptoms and how one develops the mental disorder (through genetics, environment, etc.).
2. Read stories about people who have been diagnosed with it
This is where forum and blogging sites like Tumblr and Reddit might come in handy. Compare your experiences with people on there. Obviously, not every story is going to be the exact same, but try to compare and contrast your own events and other people's, including both professionally diagnosed and self-diagnosed. Read about famous people who have dealt with the mental illness, and try to acquire books on personal accounts about the mental disorder.
3. Try to find something from the DSM-5 about it
The DSM, or The Diagnostic and Statistical Manual of Mental Disorders, is what almost every therapist, psychologist, psychiatrist, etc. generally uses as a guide to diagnosing someone with a mental disorder. The DSM-5 is their most updated manual. I wouldn't suggest buying your own copy of the book, but finding an excerpt on the Internet or checking out a local library to see if they have anything on the DSM-5 would be a huge leap in helping diagnose your own mental illness.
4. Get a second opinion on it
Go to a trusted family member, friend, or co-worker and ask their opinion on it. They could offer valid insight into the self-diagnosis, or perhaps direct you to someone else who might have better information/opinions over the matter. If the disorder is usually developed through genetics, a family member might be able to tell you family history with the disorder. I would recommend this only if you know the person you're asking is supportive and trustworthy, however.
Different people find out about their mental illness in different ways. No one shares the same story as me, and no one is going to share the same story as you. It's fine to be self-diagnosed.