I agree that riding hard outside is the best way to acquire this mental skill. But I also advocate hiding (or ignoring) interval time/duration in hard workouts. You might be surprised how challenging an interval can be when you don’t watch the seconds count down.
Even if you know the interval is 2 minutes long (or any other time)… it can become more mentally challenging when you can’t or don’t watch the countdown. I have employed the “blind” method after having similar issues as the OP.
It’s easier for me to tackle something when I know exactly how long the suffering will last. The unknown of group riding and racing is great, but we can work a bit towards that with more deliberate work on the trainer, to ignore the time.
This works best with ERG trainers, but can be done in Resistance if you find the desired cadence for the power target, and just watch that during your effort. I highly recommend that people give the “blind” approach a try and see if it helps in this regard.