what should I do my boyfriend has trust issues?

I’ve recently started dating this guy. we get on like a house on fire and you can tell he really likes me. his ex is abit obsessed with him as she went of to uni and now is back for the holidays, basically she does not like seeing him happy, although the reason they broke up is that she cheated on him several times and thats how they ended. I was with my ex like 7 monthes ago and we ended on good terms, however when we were together in the 3 years we were on rocky ground and I unfortionly kissed someone else although at the time I was very young and nieve and have grown up a lot! anyhow his sister knows everything that happened between us and shes good friends with my new boyfriends ex. basically the ex has been scopping around about me and has found out about everything, however as my exs sister doesnt really like me very much anymore I can see her making out that it was worser than it is and making out that im a monster to my new boyfriend. so I thought that I should just tell him about my past, which I did and now hes being abit funny with me. I understand why hes upset and why he has trust issues, but I’ve said a many of a times that it was the past and that I was young and I could never do that again as I see the hurt it causes and that I would never wish that on him.

I really like this guy and I know he likes me a lot but what can I do or say to just sort this crap out that really should not be happeneing I hate exs they ruin everything!!!