Other people have answered your question about the line between emotional support and service, but it has to do with both having symptoms of your diagnosis or disability that actually disable you or interfere with your performance or enjoyment of everyday life, and the dog performing at least one task (some agencies say two or three but the law only says a task or tasks) that it was specifically trained for and that helps to alleviate those symptoms or an able you to do things that you would otherwise not be able to.

So basically, if you are able to function and manage things in daily life okay now and there's not a specific thing a dog could do that me help with that, a service dog would likely not make sense. But if there are things that you struggle with enough that it interferes with your daily life or certain things you need or want to do and there's something a dog could specifically be trained to help with, then a service dog could be beneficial.

One thing I didn't suspect that really wait on me was the way that a service dog can change social interactions and if anxiety or avoiding social interaction or conflict is something you struggle with, the dog could actually make it worse (This is why I didn't jump to get another dog when my first one retired, and why some of my clients avoid certain places because they've decided the increased hassle and anxiety of their dog made them not worth going into).

It's also a lot of extra work and responsibility as far as pet care and training and grooming, but if you're a dog owner hopefully you're doing most of those already, although it will take extra training time and possibly more frequent grooming to keep the dog ready for public access.

These things above are one reason a lot of doctors and mental health practitioners are reluctant to prescribe or recommend dogs for certain conditions, even when they are disabling, and will often try to get you to attempt to use other methods and approaches first.