I'm expecting you all to be brutal and honest - don't let me down!

I'm currently studying my MA and am now looking into doing a PhD. It's a fairly new idea for me, because I've taken everything pretty slowly so far (took a couple of years out before my MA). But I can't figure out if I should do it or not.

Firstly, I'm an introvert with shyness and self-esteem issues, although I have had my confidence boosted MASSIVELY over the past few years and am now fairly good at public speaking. Secondly, I've struggled with my mental health throughout my student life, including the couple of years of work in between. Thirdly, I don't have a solid idea for my proposal yet, and feel I'm approaching this all way too late!

I'm generally concerned that I won't be a good fit for the course, although I'd like very much to do it. I want to progress in my field, hone my craft, and eventually become a lecturer or work in academia. I want the support that the 3 years would give me. I would also rather do it now, considering I'm mid-way through my 20s and would like to settle down sooner than later. But I'm not an outgoing student, I'm not massively confident in my own abilities, and I'm worried I won't be able to cope with the extra pressure (although I don't want my mental health to rule my life).

I'm currently doing loads of research and brainstorming for proposal ideas, but there's still this doubt in the back of my mind. Is this uncertainty a red flag to begin with? Should I keep on anyway, apply, continue talking it over with my lecturers? Or should I accept that it might not be for me?

Any help or advice would be very much appreciated!