How do I stop overthinking everything and get over insecurities?

Hi! TK Welcome to the BetterHelp platform.  I hope your week is off to a good start.  I'm Maya, one of the therapists here on BetterHelp.  I specialize in relationships and breakup dynamics.  In reply to your question:

How do I stop overthinking everything and get over insecurities?

Well, those are two separate issues, but I do see how they could feed into one another. How to STOP thinking/feeling/behaving in certain problematic ways, and how to START thinking/feeling/behaving in better more functional ways... that's a great question, and there isn't just one answer!   There are many ways to accomplish CHANGE.... it really depends on which therapist you ask or where you go hunting for self-help guidance.  So bear in mind that my answer is just one of many possible good answers.  

I think it would be helpful for you to see that you've got two different challenges here: Overthinking, which is generally related to ANXIETY.  And 'getting over insecurities" with is generally related to LOW SELF ESTEEM, which can sometimes (but not always) be related to a painful past experiences.

How to get over Overthinking/Anxiety?  Again, there are hundreds of ways!  A good place to start would be MOVING MEDITATION which can gradually help you gain control of your own cognition (thoughts).  You can Google Moving Meditation to explore how to do it.   Doing something pleasurable or productive when you catch yourself overthinking can also help your thoughts get "unstuck."

How to get over Insecurity/Low Self esteem?  Again, it could depend on the probable cause, and there would many ways to do it! But just in general, I would say that the two main "ingredients" of high self esteem are SELF RESPECT and SELF EFFICACY. You can attend to self respect by paying attention to all aspects of SELF CARE and by spending your resources (time and emotional energy, especially!) in ways that bring you a good return on your investment! You can increase your sense of self efficacy by breaking down challenging tasks and goals into baby steps, so that when you accomplish each one it will empower you to take the next step and then the next, building in confidence. 

And of course, consider getting into some therapy for a lot more tips and ideas! 

Well me and my partner (well ex) are on a break at the moment because I overthink everything and get angry over the little things like her going out to a concert or going to drink without me.

That may be a good insight on your part, but there are usually many reasons for a breakup or a break, and some of those reasons may be hard to notice.  So don't put all the blame on yourself.   

But I've also been cheated on multiple times and don't really know how to trust again..

I can help you with that issue if you choose to begin therapy with me.  There are specific ways to build or rebuild trust in a relationship.  Committing to being radically honest with one another at all times, for starters!  

I'd suggest you consider couples therapy too! 

Take care! 

Maya