You countered well out of your defense, though you winged a bit much.
It's apparent that you were getting very frustrated as he was tagging you in combination, and anytime he kept you at bay with a spinning kick. If you want to increase your mental endurance a bit, I'd recommend some boring, repetitive 3-minute drills. It'll help your technique, and fine-tune your focus. The goal is to engage the drill for the entire 3 minute round. Don't disconnect from the moment like you do at 3:35. If you need to "reset," do so in a responsible way: circle out, skip back with your hands out, "skip out," etc.
Overall, you need to embrace the "subtleties" of the game and have some patience. You can't rush a haymaker: it's risky, low-percentage, and wasteful with your energy. Spend some training time on more cost-effective strikes, and forget the idea of power punching for a few rounds a day. Get into probing "throwaway" punches to set up your big power, and try to observe a 3-to-1 rule where you set up with 3 probes before you're allowed to drop one haymaker. It'll help you train a more realistic approach to fighting at this level, and stop you from walking into shots unless your opponent really takes the time to set you up.
When you're being defensive, remember to use your whole body. That could mean moving your body to block or slip, or stepping out entirely. Around 0:33 you jab and then try to cover up, but your posture, hips, and shoulders don't change at all. Your head turtles and your guard comes up, but it's not very effective: you take your eyes off of the opponent, and give them a static target. That's what allowed him to line you up for the spinning back kick finish. Watch your static blocking position at 4:15; you gave him no variables to adjust to, which gave him time to take aim with a risky, high-reward technique.
Instead, you want to use small, energy-efficient movements that alter the position of your entire body, move your head offline, and take away certain attack angles. Moving your entire body is defensively sound, less tiring, and more threatening to your opponent, who assumes you're looking for an attack angle with the shift. At 0:35, you move out at a small angle with your guard up and the opponent IMMEDIATELY backs off. Because it comes right after a flawed defensive reaction, it makes for a good contrast between right and wrong - watch 0:33-0:35 a few times. It happens fast . Go forward to 0:38 to see another example of static "isolated" blocking you should avoid - isolated in the sense that you're only using your forearms, not your entire body.
