Hey Milo......
I think I'm in your position +4 years!! I am handing my EngD thesis in next week and I know exactly how you feel. However, don't make the mistake I did- after my first year it was clear that my academic or industrial supervisors had no real interest in my project as there were no clear aims/ objectives and no money in my "research budget" but I thought it would work out......fast forward 3 years and I was still in the same situation. It's literally only in the last 6-12 months where things seem to have come together, but I'm afraid it's probably too little too late.
If you have a difficult supervisor as I did, you need to force someone to take responsibility for your project (or at least help). Everyone who already has a PhD seems to find it easy to say "it's your PhD, it's your responsibility". What a load of crap. I don't believe anyone can do a successful project entirely on their own. How many industrial projects would go ahead with inexperienced employees taking the lead with no support?

Do yourself a favour and get your project aims and objectives sorted ASAP, and I mean in absolute concrete, otherwise you'll end up wondering where 4 years went. Speaking from experience it's not a nice place to be! Sorry to be negative, but I wouldn't want you ending up where I did. There's a lot of other things you could achieve in 4 years instead of stress and worry over a qualification which isn't all it's cracked up to be.

Good luck!