Communication is always the most important factor in a relationship... i know how painful it feels for things to result this way.
Sorry to say this, but i kinda feel that he might not be the 'One' you have been looking for. 'Cos i believe if someone treasure you, love you truly, he/she wouldn't find comfort in someone else's arms, as for your case, he could have talk to you about the problem.
Please remember this, in relationships, there's NO SUCH THING of being number 2, you are who you are, special in your own way, sorry to say this, but those jerks who inflicted the pain on you are immature, as to them, having 2 girls in their arms at one time give them the pride in their circle of friends.
As for you changing, it's normal for anyone who has experience heartbreaks, you just wanna hold on to what you have and not lose it again, defensive i should say... but i do have a question to ask you, do you yourself know or have the doubts that he will make the same mistakes again?
Though he reassures you, but you just feel paranoia still, that's because you hadn't got over the past, i guess basically because of your ex bfs. Forget what happen, forgive their immature thinkings and get on with your life if you want this relationship to work out.
Lastly, i really suggest you two to have a good face to face talk, about your fears and problems, it's better for you this way, if that is, eventually you think/wish he'll be the one to spend the rest of your life with you.
Good Luck and Take Care.
P.S: U remind me of someone i know...
( ._.)(._. ) *meow meow* ( ._.)(._. )