You are adding co2 to the beer before carbing with mixed gas; this is why you aren't getting a good pour. This happened to me when my blow off tube got blocked once. You need to hook up a blow off tube to your keg to de-gas it over a few days. Then carb up with the mixed gas at the correct psi for the temp of the beer. Someone previously suggested a chart for this.
Now this might not completely save your current brew but you should see an improvement. You can send the mixed gas down the liquid tube if you want to sample a pint after a few days. Take care with the disconnects though as a black one will stick on a normal gas post.
But before doing all that, you could try starting your pour by pushing back on the tap until it runs black, then pour forward. It's an old barmans trick when Guinness is running high. Finish the pour by pushing back. If the head is a bit big, FLICK some of the head into another glass, don't try to pour some of it into another glass as you will only end up pouring beer too. Top up again by pushing back. Don't throw away the flicked head as this will turn into beer and can be used to top up your next pint.
But next time, don't add co2 before carbing with 75/25 and you should be grand so long as you set the correct psi. I have mine set at 32 for approx 7 or 8 degrees C. From memory that is about 1 vol of co2 but double check that.
