I do not have many details about your relationship. Because of this, I can only provide a speculative answer. There are two issues. One is that you cheated on your ex-boyfriend on three occasions and you are not sure why. The second is that you do not trust your ex and the reason for the distrust is unclear.
You may not be sure that you want your ex. Perhaps you like him but he is not your “first choice.” As soon as a “better choice” comes along, you go in that direction. Perhaps there are other reasons but I can’t determine that from your letter. A thorough discussion of your circumstances with a therapist would provide deeper insight into the reasons that led you to cheat.
The second issue is that you do not trust your ex. This might be because you heard that he was with another girl. Another possibility is that you may have difficulty trusting people in general. If I interviewed you in person, I would want to explore whether your trust issues extend to other relationships. If you harbor a basic distrust of people this might explain why you have difficulty trusting your ex. Trust is something that is earned. Has he given you a valid reason to doubt his trustworthiness? If not, then your accusations are likely unwarranted.
There are also situations where an individual is cheating, yet they accuse their partner of cheating. This is called projection. Projection is an unconscious psychological defense mechanism. It involves taking unacceptable qualities or feelings about ourselves and ascribing them to other people. Its purpose is to reduce anxiety.
If you are having trust or relationship issues it might be best to seek counseling. This is especially true if your desire is to reconnect with your ex-boyfriend. Understandably, it may be difficult for him to trust you again. You have violated his trust on three separate occasions. If you would agree to counseling, it might demonstrate to your ex that you are serious about changing your behavior. It might be one step towards earning his trust. If you would like to find a therapist, you may want to try this website. A therapist may be able to further clarify your behavior and also teach you how to engage in healthy relationships. I would highly recommend it.