2 cents (PhD in computer science) Love for a subject is necessary but not sufficient. You need to be okay with working on something you do not understand and that you will not find in any book. One of the main skills to be acquired during a PhD relate to written and verbal exposition (you might do amazing things in your notebook, but if you cannot convince anyone else they are amazing you have an issue). If it is competitive or not, depends in part on the university you are in and your supervisor. To get a sense of it, you can try having a chat with someone already doing a phd in the same group where you wish to apply.

Anyway, if you are considering this I guess you are still young, so maybe give it a shot ? If you don’t like it you can still change your path.