Developing an adult game is something I, and many other devs do as a hobby, for fun, and not to make a living. We put a lot of time and effort into the whole thing even without it being our job and while having a RL job. With that comes the problem that you simply can't predict everything, you can't plan everything, you can't always take the time you need or want.
Especially if you work alone, there is a lot you can't foresee.
This might be different if you have a team where others can jump in, or if you do it as your main job where you can make your own specific working times.
There is a reason so many dev's give up. One reason might be that people simply misjudge how much work it actually is, but as a dev there is always a lot of pressure on your shoulders, even without a sheduled release, it's always there and this can easily burn you out.
Talking about professional developing, many (almost every) of the big game studios delay their game releases several times because they simply can't make it in time, and still, nowadays many tripple A games are released bug filled, and unfinished.
If that's "professional", I'd happily stay away from it.
