I have thought

[AirbusA321]

Are all these versions correct?

1 I have thought/have been thinking about this problem for a long time, yet I haven't found a solution so far.
2 I have thought/have been thinking about this problem for a long time but still didn't find a solution.
3 I have thought/have been thinking about this problem for a long time without finding a solution by now.
4 I have thought/have been thinking about this problem for a long time but still can't find a solution.

 

[bhaisahab]

1. and 4. are good.

 

[Rover_KE]

Conversationally, we'd start by saying I've ...

 

[AirbusA321]

Does it make no difference whether I use "have thought" or "have been thinking" in the first part of the sentence?
Would sentences 2 and 3 be correct if I omitted "still" and "by now"?