>>1491827maybe look at your diet, get a lot of protein and carbs, and drink plenty of water
https://www.youtube.com/watch?v=zuY6M3xEqvIthe tires are slowing you down a bit but it's not massive
maybe check your saddle height, measure your pubic bone height (shove a thin book hard up your crotch) and use the 107% or 109% method as a reference
strength training in a gym or on your bike might help not just in the muscle department but in the mental aspect as well of how to push harder
