Car dies when you put it in gear

Could be a lot of thing's , you should have it checked at a qualified repair shop .

Is the check engine light or service engine soon light on ?

Just a suggestion here but my son was experiencing the same problem with his Toyota Celica. We replaced the clutch and throwout bearing and MC, had me stumped for days the I pulled back the rubber boot on the MC where the it comes through the firewall under the dash and saw that there was fluid leaking into the boot so basically the MC was bad an it was bypassing the metering valve causing the fluid not to push the slave cylinder all the way down for engagement. On another car ...a 97 Saturn SL it wouldn't shift into gear, also all new clutch components. Wound up being a bad top right side motor mount was bad. engine was sagging just enough to cause the clutch not to shift properly. I hope this helps.

Stuck throttle linkage or bad throttle position sensor. Also possible is an air leak on the Intake manifold. What happens is that you allow a certain amount of air to come into the engine when the throttle cable is pressed. But if you have an air leak, additional air is coming in the engine and it is like you have the throttle opened further.

You should have the engine shut off and then look at the throttle cables and work the controls. They should operate easily and shut smoothly. If they do not work smoothly there is a problem.

You also can check the gaskets on the intake manifold by using an old spray bottle and mist water onto the seams of the top of the engine while it runs. If the RPM changes or the engine dies, you have found a gasket to fix or tighten.

The engine should not idle so fast as to cause you to ride the brake hard to stop the vehicle. Autozone, Advance, or Oreilys autoparts will scan your car for free. Sometimes they are reluctant if the "Check" light is not on, but revving up may spur them to help you.

Check to make sure that one of the pulleys are not sticking on your serpentine belt line, or better known as the belt drive. Such as an alternator that is that seizing up, or a bad power steering pump.

Did you check the idle air control valve? See tips at the link below.
Do It Yourself Diagnosis and Repair

I cannot see how adding a fuel additive would help to be honest. I would be looking for a split or leaking vacuum line, I have even seen a leaking evap canister before that caused a similar problem. Does the engine pull clean and strong when it is warm or at cruising speeds? Does it feel like its running lean or misfiring?

coolant temperature sensor boken, check and replace if defective.

Idle air control valve is a common problem Might check that first. Is it throughing a check engine light? with that I can help you further.