Hello, just a quick novice query regarding an issue I have with my commuter bike (a fairly new Specialized (brand) hybrid)

I have been commuting to work for a few months now but I feel that the cycling is overly tough going up hills/inclines ie. harder than it should be - this is making me lose motivation to get onto the bike and makes me dread the ride home.

I am a relatively fit person who plays sports at least 3 times a week and can manage quite long distances running but I am quite a heavy/large build at 108kg. I mention this because no matter how much I blow up my tyres they go a bit flat when I sit on the bike - could this be the cause of my problem or is this normal when seated on a bike?

The tyres are quite thin road tyres and came with the bike - is there a type of tyre better suited to larger people which would hold its shape? I really feel like something is slowing the bike down and I can only think of the tyres as the reason other than my weight.

Any help much appreciated!