OP, your situation resonates (if still sadly unrelatable on account of me not having the £3m in the bank that you do ;) There are enough trust fund babies & high net worth entrepreneurs who've gone before you that have faced your same dilemma. I'm sure you can apply yourself to find some best in class guidance on how to resolve a worthwhile path.
Plenty of decent advice on here, but putting others ahead of yourself & figuring out what you want your 'legacy' to be has got to be at the core of getting some of that original motivation back. Still no small mission to figure out WHAT that spark will be, so I wish you all the best figuring that out without spiralling to drink/drugs/depression/ other destructive behaviour which is also so common as a path for those born into this much monetary reward without any effort needed...
I was taught that a proper rounded education includes:
academic - you seem to have sorted out intellectual achievement & extrinsic reward fairly well already.
physical - seems you're on top of this daily. I'd back up others saying freeze some eggs now though, it buys a lot of options against all sorts of things that may happen over the next 15-20 years. Some people go all out on intense adrenaline sports & the thrill of the chase to get that early risk/motivation rush again, but you don't technically have to take up base-jumping or wing-walking. Yoga & some time in the weights room is plenty for long term health ;)
creative - this seems to be a missing piece for you. Perhaps setting up your business fulfilled this element previously, but there's plenty of science behind the need to create something new to keep the human brain ticking along in a well balanced way - whether that's music or writing or sewing or flower arranging or whatever other outlet you can think of, pick something & start doing it. You don't have to be good at it, just choose something & start.
spiritual - again, plenty of research that shows that humans just have a void if there's nothing filling this. Doesn't have to be religious, humanist is totally legit, but ties in with the sense of purpose/philosophical why am I here thing. Lots of people use meditation just to stay in the moment & not spiral out on the existential 'what next' route. That can help. Others look at buddhist/ayurvedic/daoist type religions which can be a bit less proscriptive about how to live - or more, but in a much less western way. No-one else can tell you exactly which path will help you find your peace here, but sound a few out...
community/charity/education/giving back - plenty of people have suggested this is where you'll find answers. Given your background, understandably most of your path so far has been focused on overcoming your upbringing, achieving material/financial success & attaining a level of security you never had growing up. But now you've got that 'empty/unmotivated' feeling loads of the trustafaraians also find themselves with.
Again, plenty of research showing that 'paying it forward' is the most likely way to bring satisfaction & fulfilment to your life now. It doesn't sound like you spend a lot of time with young people. MSF would be a good option given your technical skills - and offer you a path to complete your training outside the strictures of the NHS.
But you could also work with the CAB/small business bank/charity fund or grant allocation committees/business skills outreach to schools/university technology accelerators/angel investment funds to apply your proven skills to help a new generation. Which level may depend on how local or global you want to be about it, and how happy/altruistic you are about the wealth you already have vs whether you want to use your skills to invest to increase your yield or simply to educate others etc
The advice to take a year away isn't bad either - you have the means. By all accounts you can't have taken much time 'out' at any point in your life so far. You probably have all manner of emotional & physical stresses locked up in mental patterns & how you hold your body. Do a retreat. Do the somatic releases. Do the CBT/trauma reconciliation. Find a good coach who's taken millionaires along this path with sensible outcomes before (rather than some dodgy svengali who will relieve you of all your cash).
There are no easy answers & no-one can tell you exactly what to do - the balance you need still has to come from inside yourself. But hopefully the above might give you some worthwhile pointers on where & how to start?
Good luck, firmly in the category of 'nice problem to have' for most of us, but still needs to be taken seriously by you so you don't unravel. Hope you can enjoy the journey 🤞