What's wrong with my shower?

Me and my parents shower water run through the same pipes, but the shower heads come out on different sides of the wall. My parents seem to have ample amounts of hot water, I only get 10 minutes if I'm lucky. When I take showers, I start out with the water barely warm enough. After about 7-8 minutes, it starts... show more Me and my parents shower water run through the same pipes, but the shower heads come out on different sides of the wall. My parents seem to have ample amounts of hot water, I only get 10 minutes if I'm lucky.
When I take showers, I start out with the water barely warm enough. After about 7-8 minutes, it starts to get cold. I turn it hotter bit by bit and get these 30 second blasts of warmer water, until either the faucet is turned as high as it will go, (and barely warm) or it just plain runs out of hot water. I have to rush through my showers and most of the time have to finish them in cold water. I took a shower in my parents shower last night, and I stood under nice warm water for 25 minutes. (I don't normally take showers that long, I was testing it.) What could be wrong with my shower? Is it the faucet or the shower head? I know we should have someone come look at it, but we're do-it-yourself-ers, so we don't want to spend money to have someone come and fix something we could fix ourselves.
Update: @Vanessa - It is softened water through culligan
Update 2: @Just Trying to Help - Lol I know nothing about this stuff so I'm pretty gullible :)
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