I've just found out my PhD application was unsuccessful and I'm not sure what to do next. I'd previously applied to funded projects following my masters degree and was unsuccessful, so I've worked hard the last few years to gain skills and experience. I now have over 3 years experience in research roles, three publications (2 of which I'm first author), I've presented at 2 major conferences (third in June), and in my current role I've secured over £200,000 in grant funding and I supervise master's students.
I worked with potential supervisors on a strong project proposal and applied for a funded studentship to conduct this project, however following interview I was unsuccessful. I have asked for feedback to try and figure out why but not had a reply yet. I don't know what to do next as I don't know what else I can do to be a good candidate, or if maybe I'm just not suited to PhD study. Any advice would be much appreciated.

I would suggest a different approach. Get a job with a major company, work there as an industry researcher for 1-2 years and then ask for them to sponsor you to further your study.

That way you have work experience and may have the opportunity to do a PhD and still have a job to return to at the end of your study. Plus, you may have a better idea of what PhD project to work on with industry input.

Just keep trying.
PhD applications are a lottery.
I once applied for a PhD in science at a UK university. The application was unsuccessful.
I then asked for feedback. Their feedback was along the lines of me being a very cute kid (!!) - and that is about all they said - even though there hadn't even been an interview, and the staff hadn't met me either in person or online. I am still keeping that email as evidence.
I am aware that many people give advice along the lines of getting more experience in either research or industry, publications etc so that you have more chances of being accepted to the PhD. My experience was very different.
Other experiences you may have is that you apply for a highly competitive PhD - one that asks all applicants to have a first and possibly other requirements - and then you discover your supervisor is an incompetent who doesn't know the science they should know in their position.
It shouldn't be this way but that is how it is.