I got fired from my job. Advice?

I started working at a retail store 6 months ago. I've always been the youngest worker. Since I started the rules have always been a double standard. I would always get yelled at for walking off my register while another associate who should be on theirs walks right by. I was always on time and always did what I had to do. The GM even said I was his best worker. They gave us rules and I would always follow them while other associates didn't. I would take regular breaks while other associates left for hours but once I'm at my 46th minute the managers would bust in the break room & ask why I'm not clocked back in… I felt I was harassed most of the time for little things like going to the bathroom & when other associates did that I would have to handle the entire line alone and it was no problem. If I stepped off it was the end of the world. Even the newer associates got treated with more respect than I. I'm quiet and don't speak up a lot, but I started to get tired of the bad treatment. It made me very depressed & I started to hate that job. Every associate noticed how bad they treated me too. School got in the way of my schedule at times but at least if I couldn't come in to work I had a legit reason (and doctors note). After a while I lost a lot of motivation to work there. Last weekend I didn't come in and got fired for it (was sick). They claim I call out too much and my attendance is horrible (a lie) I asked him to show me the time clock and he refused.
Added (1). Other workers use their phone at the register and come in hours late all the time. I lost all motivation when I went to speak to the GM about how I was being treated and he began to treat me the same way. Everyone that worked there was in disbelief when they heard I was fired. They all knew I work the hardest out of all the cashiers and am very respectful unlike most of them. I'm even in disbelief that I was fired but at the same time not considering there has always been a double standard
Added (2). I'm actually relieved that I am not working in that depressing place anymore. The only thing that upsets me is that I was FIRED so that looks bad. (I was going to quit but I needed the money because I just bought a used car) [ 2002 honda:)! ]
Added (3). You would think there is another side. But there isn't. I was treated differently from the start but always worked my hardest. There is no other side and that is why it's unbelievable that I was fired. I LOVED that job, I HATED how the managers treated me.

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