Problem in Thevenin voltage
I've redrawn your schematic a little. I made a choice about which node to call "ground" (you get to do that for exactly one node.)







simulate this circuit &ndash; Schematic created using CircuitLab

If I followed your question, the dashed box surrounds the part where you need an Thevenin equivalent.

Let's continue the redrawing process just a little further. The next step is on the left and then a sort-of giant step upon moving towards the right side:







simulate this circuit

You should be able to see how I got to the left side and should also be able to see why \$R_5\$ is irrelevant.

What is perhaps just a tiny bit more subtle is that \$R_1\$ and \$R_2\$ have also become irrelevant. They don't affect the current source in any way, at all. Neither does \$R_3\$, because the current flows through \$R_3\$ regardless of its value.

So the right side now shows you a reduced circuit that is quite a bit easier to apply yourself towards.