It depends on what your goal is. For example, if you are wanting to understand yourself, and those conditions describe your experiences, then you can find comfort in relating to those traits and having a way to communicate your needs to others. Or, if you are looking for a support group or community of people whom you feel understand your experiences, then self-identification can be helpful. If you choose to self-diagnose, you need to be completely honest about that with others so that they are not misinformed and to remember that you cannot officially diagnose yourself. Self-diagnosis can be understandable, as many people either don't have access financially to assessment or they wish to avoid stigmatization or discrimination that could happen if their diagnosis were officially documented.
However, I strongly recommend seeking expert advice, assessment, and diagnosis, as this is more credible, can lead to greater supports (like medications, accommodations, or programs), and could rule out differential diagnoses or reveal concurrent conditions. Of course, even expert diagnosis cannot provide you with the security of 100% certainty, as assessment is typically ongoing rather than a one-time deal, clinicians are human so they can get it wrong, and the scientific field is constantly learning new information about mental health conditions (hence Diagnostic & Statistical Manual revisions).
Differential diagnoses are conditions that can present like each other, but have distinct differences, even if subtle. For example, I work with autistic adults who were misdiagnosed, late diagnosed, or underdiagnosed. Autism can at times present like Borderline Personality Disorder and Avoidant Personality Disorder due to the social anxiety, communication difficulties, and difficulty interpreting others' motives. Another example would be PTSD, which could look like Borderline Personality Disorder due to anger outbursts or feelings of social disconnectedness.
Concurrent conditions are simply conditions that are present within a person simultaneously; and often, certain conditions tend to co-occur with one another. (You will also hear the word comorbid used interchangeably). For instance, in my work with autistic clients, I have learned that ADHD and OCD commonly co-occur with autism due to neurodevelopmental factors. Another example would be that anxiety and depression often co-occur.
Even still, there are traits that can look like conditions but that are actually divergent yet subclinical without causing impairments in functioning. For example, a highly sensitive person who is gifted & talented could seem in some ways to be autistic or ADHD due to sensory sensitivities, hyper-focusing on interests, or social awkwardness, but these traits may not meet diagnostic criteria or may not interfere with the person's work or social life in a way that causes disability or requires supports.
Assessment involves a number of factors, like ongoing therapeutic observation and relationship building, screeners, interviews, scales, cognitive tests, history-taking, etc. Formal assessments can take anywhere from one to several days, while informal assessment can be indefinite.
Lastly, clinicians differ on what they believe about diagnosis. Some believe that diagnosis is harmful because it puts people in a labeled box that they can either use as an excuse or interpret as inherent defectiveness. Others believe that diagnosis is crucial to forming a treatment plan and helping a person gain the self-understanding and awareness they need to work effectively on their struggles. Other clinicians (such as myself) fall somewhere in between, believing that having a name for one's struggles can help them progress forward as long as they don't use it as an escape. I know this was a detailed answer, but I believe that's what people deserve because they are hungry for understanding! Just remember, diagnosis is like a sword; it could be used well or misused. Professionals are best equipped to wield it, but laypeople can learn from it.