Help please, I need advice.):

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I know that this is just the internet, however I am just looking for some advice on what to do with my current situation.
I recently accepted a job offer from a major company in my area and declined another job offer from another major company. But the last few days before making the decision I couldn't decide which company to go with. Now that I've made my decision I feel like I've made a big mistake. But at the same time, if I think of it the other way, I feel like I'd be missing out on a big opportunity. I can't seem to make up my mind on what was the right decision. It's been causing me to have trouble sleeping, I feel like I want to cry nonstop. I wish I could work for both companies but I can't since they both want to hire me for full time.