I need advice, anything that can help

(Sorry if this is long) I'm seventeen and I'll be eighteen in October. So in the past i missed/skipped school alot and got sent to court, causing me to owe over a four hundred dollar fine. It was towards the beginning of 2015. After that Summer i started going to a place through my school that does online schooling and since then my grades are great, I've been attending. The judge told me he would lock me up though if i don't have the fine paid by the time i turn eighteen. So my first job i got i had gotten fired. I guess i wasn't "enthusiastic enough." I got paid once, only lasted a few days. So i didn't put the money on the fine. But anyways, now i have gotten another job offer tomorrow. And i must say, I'm a bit worred. I'm afraid i won't be able to keep the job. I just got out of a long term relationship also. We still tell each other we love each other and it's just really confusing. But I've been depressed for the longest time because of these things. I haven't been eating much/lost lots of weight, I've been sad and crying. I also been feeling like i have social anxiety. I feel like i won't be able to do my best at this job because of this. And the longest i worked at my last job was about maybe four hours. Two hours in and my back was always killing me and i had to hide it. Which is why i dreaded going to work. This new job I'm supposed to start six and a half hours. I'm not used to standing all day and i hate pain. I just don't know what to do. I don't know how to push myself and tell myself that everything is okay......