Its not in your head its fact. Lets go with the reasons why it feels it is and actually WHY .....
HEAT/WIND....
Training indoors, then get a fan and good air movement. The air circulating the body has been shown to lower skin temperatures when riding outside. Inside cooling is lower which leads to an increased sweat rate, leading to dehydration and cardiac drift or higher heart rate in relation to effort. All leading to a feeling of increased exertion....
BODY POSITION.....
Riding outside we are adapting our position to what's happening on the road with cornering, climbing, descending, braking and free wheeling....All these micro changes help to relieve muscle tiredness and body stiffness, not to mention the point of where we are in contact with the saddle !!
We generally use one to two body positions with a hoods/bars/drops combo and plenty of time in the same position. This leads to rapid fatigue with muscle fibers being used for longer periods of time. Outside we spread the load easier also with out of the saddle bike movements.
ATTENTION/FOCUS.....
Riding outside time passes quicker as we focus on changing scenery, breathing, road ahead, daydreaming even what's happening on our device. Riding indoors time passes slower we are purely focused on the intervals and what we are doing. Virtual riding helps us to disassociate ourselves with what we are doing. One reason for the popularity of this medium the last few years.
WATTS/POWER DIFFERENCE....
Generally as Coaches we work on a +/- 20w difference with inside/ outside...PERSONALLY I would not worry tooo much about that unless doing long periods of inside in relation to outside in which case have two FTP's to cover your bases. (Remember FTP is a range not a number so dropping it down a few % will help you out !!) The reason for this number is that studies have shown that over time with athletes training inside and outside they can produce somewhere between 11 to 23w difference when outside.....
SO TIPS TO GET BETTER AT INSIDE TRAINING .....
*Stay cool with fans and good ventilation,
*Move around as much as you can with your position both standing seating AND drops/bars/hoods
*Stick some music on/video/get on Zwift/Rouvy/Trainer Road and go virtual
*Adjust your FTP or be prepared to suffer a bit to hit your targets
Indoor training can give you some SERIOUSLY focused sessions when the weather/time is short so if not a fan of indoor use these tips to think about how to improve your focus !!!