Why is it so hard to surpass speed of light

So in theory a body can't travel at speed of light unless it has 0 mass or infinite energy to provide acceleration. If we look at the space shuttles we've built we can see how ridiculous they are, they have fuel (which increase their weight) thus increasing the mass and the force needed to accelerate. and of course they are limited in speed. now.....what if we have a infinite energy to provide acceleration without requiring fuel? some sort of magnetic engine...it is not that hard to create one....and if we look at our available Technology closely...we have everything to create one. So lets suppose we do have a magnetic engine that provides some sort of energy proporcional to the shuttle radius with no need for fuel? wouldn't the ship accelerate until the speed of light and eventually easily surpass it? Of course the time of acceleration would be dependent on the quantity of energy provided...but eventually it would reach speed of light...no matter how long it would take. the only problem I see with this....is that the molecular bounds of the body/shuttle wouldn't probably survive that speed. but anyhow....we don't need to travel at the speed of light to reach mars in a day or two....no need to 20 years. So my question is...if recreating this conditions I mentioned (which seem very possible to me) would it be possible to travel at faster speed (maybe even speed of light) and why on earth have we never tried it? I know voyager uses solar light to accelerate....it almost there...if it had a constant source of light wouldn't it reach speed of light too?

You cannot create energy out of nothing. You can either create energy out of other type of energy or from mass, by the process of nuclear fusion which is not an available technology for now. A 'magnetic engine' is not the best option. The magnetic energy provided by a regular magnet or even a greater one is not that great, that's why we need the particles to do several turns around CERN before they reach the necessary speed even having very powerful magnets.