I constantly worry about ridiculous things, mainly things from my past. How can I stop thinking that something bad will happen?

Well positivity is the key to life, It is totally normal to think back about your past exspecially if it’s rough, surround yourself with good not Rough things

Try to see things from a positive view instead of a negative view, this will actually help you so that you won't see things from a bad point of view

You are not alone. They are called Intrusive Thoughts, and they are very common, especially in people with Anxiety disorders. I used to get Intrusive Thoughts all the time. Some people will visualize the memory or the thought as an item, then destroy the item, or imagine the item disappearing (like a puff of smoke). Being able to physically "get rid of" the thought really helps some people. Another thing you can do is to block them entirely, just not allow yourself to think the full thought out and instead think of something else, like gardening or yoga or astrology.