If you read your own post it does not seem to make sense. You are 20 so cannot have been 3 years in college and employed for nearly 4 years unless you are talking about day release course. Next it cannot have taken you 6 years to get where you are seeing as how you could not leave school until you were 16.
You are already facing the reality that so many car mad lads [including my youngest son] have found, that the teen ager's dream of working with cars means you end up in a low paid job with negligible promotion prospects. Basically from all the trades you could choose to do being a mechanic is one of the worst paid.
You say you are fed up earning the boss a lot of money while you just get peanuts in return. It is not as straight forward as that, your boss has a very large sum of money tied up in his business. I believe is costs around £60,000 just to equip your garage to be an MOT test station.
If you think that you will make big money as a mobile mechanic you are day dreaming. Without your own workshop facilities presumably that is what you are intending. For a start you are limited on just what you can do as a mobile, mostly it would be just basic servicing, brakes etc. Whilst changing a gearbox with the car raised up on a ramp is fairly straight forward doing in lying on your back under axle stands on someones wet drive is something else. You wont have diagnostic equipment, tyre balancer, tracking equipment, head light alignment equipment etc. You dont start out as a Limited company just as a Sole Trader. You dont need to be vat registered on a turnover less than 73k a year which as a mobile you would never reach. Further, as a recently qualified 20 year old you will have limited credibility as a trades person. Hard but fact. You can only do your MOT course while you are working in a garage that is an approved MOT testing station and if you go self employed the only way your ticket could be used would be whilst working ~ part time at least ~ at an authorised testing station who are willing to give you part time work and add you to their register of Authorised Examiners.
You would need both Motor Trade Insurance and 3rd Party Public Liability insurance and {and i might be wrong in this part} i don't think you could get Motor Trade Insurance at your age.
Don't get me wrong, i am not saying setting up on your own is the wrong way to go, but it is the wrong way at this particular time of your life and experience. You need some cash behind you before you start up on your own because it takes time to build up a client base. Also remember that if you c*ck a job up it is you the customer comes back at for recompense, if you have no work you have no money coming in, if you are off sick you have no money coming in, if you take a holiday you have no money coming in. But all the bills keep coming in.