Flashbacks are caused by unresolved trauma. Now there are many things you can try at home called grounding techniques for when these flashbacks happen or just when you are triggered in general. A general google search for these techniques will pop up a lot of information. Pick and choose what you think might help you. Essentially what happens is that you are getting pulled back into the trauma and reliving it, so you need to first recognize that a flashback is happening that it is not really happening now, that you are safe and then use a technique to ground. A good one is the 54321 method. So engaging your 5 senses, look around find 5 things that are blue, find 4 different things you can touch and what does it feel like, Note 3 things you can hear, pick 2 things you can smell, and 1 thing you can taste. The quickest way to ground is through smell. So keeping something that has an aroma that does not remind you of a trauma but is soothing or can evoke a good memory is helpful. If these flashbacks are continuous and life limiting it is advisable to consult with a therapist. They will have the skills and knowledge to help you process the trauma you've experienced. I wish you all the best!
First you'll have to think about them for a few seconds, because you need to understand why they're triggering, when you understand what's causing them try avoiding them as much as you can. Memories can be brought in lots of ways, like smells, an image, sounds even just in loneliness. Try doing something that you think you'll love and also that has 90% chance of not bringing back those memories, or maybe talking about them too, sometimes talking helps you get a bit less stressed about it. Do that until you feel comfortable with the flashbacks. If not and they start to get bad please try our mindfulness guide if that still doesn't work I would suggest you try a therapist and maybe try solving what's triggering them.