Trust issues are characterized by fear of betrayal, abandonment, or manipulation. And this fear is often triggered as a result of betrayal (such as infidelity), abandonment (think: leaving a child or foregoing a relationship with them), or manipulation (for example, dishonesty or gaslighting).

Like your partner cheated on you over her best friend, they both betrayed you which is unforgivable once the trust issues conquer you... it'll be difficult to trust someone again, however, trust issues are meant to be an experience for us so we already know what we're gonna do if history repeats itself. There's always a solution to defeat the trust issues that start over to: 

"Analyze Their Intentions" for you in view of that they caught your attention in order to protect yourself.

"Stop Overthinking" about the past and your current problems instead use your mind to overthink how you can solve your hardships this is useful in every problem. 

"Protecting Yourself" doesn't mean you're hard to get you're just DEFENDING yourself from being damaged as the Carpenter said, "You've got to love for what I am".

Always remember my words "One person can change everything. And you just have to open up to them. And it will take a long time but, it will be worth it."