Discouraging HIIT

I can't help, but after HIIT I feel so discouraged...I don't even have that good afterwork feeling as I'm not able to accomplish HIIT Workouts in full form and I need to take longer breaks between repetitions. I have no idea how to get better in HIIT. I'm doing fitnessblender daily from January and although I got stronger from strength training, more flexible from pilates and stretching, but I can't see any progress in HIIT/cardio ;-( I try to push harder, but I have problem to breath...I plan to do FB Sweat program, but I'm worried about amount of HIIT there...