Should I get back together with my ex-boyfriend who has trust issues?

I'm truly sorry to hear that your relationship is causing you such distress at this time. 

When we are in relationships, trust is so integral to our satisfaction within the relationship.  Often, when we feel we are required to prove ourselves, feelings of resentment can replace those feelings of wanting to be supportive.  

In situations like this, I often recommend being open and honest in your communication with your partner.  Your fears about his response are valid, as he made an assumption which you cannot disprove because you cannot battle against a shadow fact. 

If you feel comfortable doing so, you can always ask him why he is so concerned you are unable to remain faithful and challenge those beliefs with the facts that disclaim them.  You can also ask your partner what it is that he needs from you to help you to help him trust in your responses.  

In the end, these are issues that you cannot conquer for him - you can only guide him and show him the path towards trust.  You may suggest couples counselling or that he seek out a professional to talk to, as well.  But in terms of your question, only you can decide whether you feel you can remain in a relationship in which you defend yourself against an uncommitted offense. 

Trust is essential in romantic relationships and it is common to have trust issues - because the stakes are so high. If both of you are committed and willing to be fully honest with each other, trust can be repaired and become stronger than ever. I would encourage you to seek out professional help as it easy to get derailed by fear when trying to work things out in the absence of trust. When fear takes over - we tend to become defensive and act in ways that makes it harder to trust one another. In addition to getting professional support I would encourage you to learn more about building trust in relationships and deepening connection through accessing various online resources on relationships.