Maintaining a constant current with varying loads
There are a few other constant current type regulators to consider: -




This biases a transistor to have a certain voltage across the emitter resistor and this means the current in the collector to the load is pre-defined.
Can be used with a LED as load - it works by the BJT cutting the voltage supply to the gate of the MOSFET when the current thru R2 reaches "the constant current level".
Classic voltage regulator turned constant current source
My favourite - an op-amp used with a BJT sets (very accurately) the voltage across the emitter resistor and hence the current in the collector is very well controlled too.
Pretty much the same as (4)
Like (2) but with the load at the top
Like (4) and (5) but with the load at the top


Also to consider are Wilson current mirrors (a definite improvement over your standard current mirror) and the Howland (and improved H) constant current source.

I don't really understand what you are trying to achieve so I can't recommend one over the other. AD have a good paper on current sources too.