All the above advice is excellent, and sound. I would also encourage nononsenseselfdefence.com. I learned several things from there that I have been able to use effectively when called for. I grew up in a rough borough in London, and while I am nowhere near as experienced or smart on this as Geoff Thompson, a lot of what he talks about is stuff that I found either let me avoid trouble, or smash trouble in the head until it stopped being trouble when it was unavoidable.
Idaho mentioned mentality, and you'd do well to pay careful attention to what he wrote.
At the moment, your mentality is all wrong, your post screams victim. That kind of mindset is a sure way to kop beatings and get your stuff jacked. Once you get your mindset right, you'll find you don't have to find yourself in these situations often, or at all.
For example, no one has tried to jack me in nearly a decade. Not cos I'm invincible, or anything close to it, but I do have a mindset that if someone, or a group of people picks on me, if a fight is unavoidable and I'm going to get fucked up at least one of them is going to be fucking lucky to see the morning. Whether that's actually how it would go when everyone got down to business, doesn't really matter, cos there are guys like you out there that make easier victims, and predators like prey, not other predators. None of this is meant to shit on you in any way, but it should show you that you need to get in the mindset of being a fucking predator.
Also on the subject of mindset, you say you cant get your motorbike back cos the guy who knicked it is way bigger than you. That to me, shows you're focusing on the wrong details. If a guy is miles bigger than you, or you can't beat him one on one in a fair fight, then all you need to do is stack the odds in your favour, weapons, ambush, catching him off guard at home/in the middle of the night. THE IMPORTANT QUESTIONS WHEN YOUR MINDSET IS RIGHT DEAL WITH THE BIGGER PICTURE, SO THINGS LIKE : WHAT ARE THE CONSEQUENCES? IS IT WORTH IT? IS HE GOING TO COME BACK STRONG OR HIDE AWAY? WHAT IS YOUR REPUTATION?
For example: if someone much bigger than me knicked my bike, the size of the bloke wouldn't concern me. However it ends up getting physical, by the time it comes to that, everything will be so stacked in my favour that I'll already have won. That's the easy bit.
I wrote a long paragraph on how I would answer those questions and what kind of steps I would take to try and give you an example, but I realise this is an open forum and I don't have any idea who might read it. You seem like a nice bloke in a rough spot. If you wanted to post specific questions that we could give you hypothetical responses to, then I'm sure you might continue to get good advice.