It's important to develop a strategy for getting through your emotions without always relying on immediate venting. The reason this is important is because there will be times when you're struggling and the people you would normally talk to will also have a lot on their plate unable to help you.
Think of a couple of things that you enjoy doing and find relaxing. I come back now to the strategy of escapism; mine has been Harry Potter audiobooks for some time now and I don't see it changing in the near future. I know that as soon as I spend a couple of minutes listening, my nerves will start to settle. It's not a complete solution, but bringing yourself back to a state where you know you have at least a little bit of control over your thoughts is the first step.
Next, if you don't know what it is specifically that's bothering you, then try to identify it (or them, there can be multiple things).
Once you've done that, think about if you can help solve any of the things bothering you.
Now, if you're in school (or at work) and you have 50,000 things due in the next 3 days and you're panicking, this is when you need to step back for a few minutes and temporarily check out mentally. Once you've passed some of the panic, be honest with yourself; if you start working again, will your spiral? If the answer is yes, then pack up, make a new schedule, and go home. Take the rest of the day for yourself, do whatever you need to do to make yourself feel better, and get a nice early start the next day. The amount of time you spend on your work is significantly less important then the level of focus and rationality you have while doing it.
If you're concerns are surrounding things like the health of a loved one, or anything that you can't fix by checking off a to do list, then this is when I would say to write it down, even if it's just "I'm worried about grandma's health", that can be it, externalizing is powerful. Once you've done that, and exercised your method of escapism, there are three possible options: 1) see if you can schedule a time (for later) to call someone, knowing that you have that set up will likely help you get through the rest of your day. Alternatively, someone on the internet has almost definitely gone through what you're going through and written about it; take a quick search. Finally, my next go to would certainly be my SHINE App. In my opinion, it's a great app for people that don't like traditional meditation because it has multiple different components (articles, daily check ins, and REAL stories from REAL people talking about their experiences.
Finally, you may have heard this before, but you really need to learn how to be your own best friend; do this through rationalization. If someone close to you, came to you with the concerns and struggles that you're currently having, what would you say to them? Chances are, you'd be very supportive and understanding. Moreover, you'd probably be a lot more rational because you wouldn't be the one panicking.
As a work related example, over the last year, I've spent a lot of time going in and out of hospitals and doctors offices for ultra sounds and blood tests, and had to deal with some new medications. As a result, I missed a significant amount of class and I had to call in sick to work far more then I EVER have to any job in my life. The whole time, I felt awful because calling in sick is always hard on everyone and my boss was always so understanding, that in a way, I almost felt worse. However, I also knew, that if I was in her position, and a (generally) very reliable employee of mine was dealing with unexpected physical health issues and missed some shifts as a result of that; I knew I would be equally as understanding as she had been to me. So with that knowledge, I thanked her sincerely every time, and I got her a nice Christmas gift and card. All is still well at that job and I've only had to call in once in the last 3.5 months because I was able to recover with the knowledge that she wouldn't hold it against me, and that if I ever end up in her position, I'll be equally as gracious as she was; pay it forward.