It is natural, do not beat yourself up over it - it is natural reaction merely because of what she put you through, in a sense, he should understand your frustration, however, you also need to take responsibility and have faith in your boyfriend. If he feels you do not trust him, it'll get him upset, thus the trust in your relationship will slowly deteriorate, which is the foundation to any given relationship.

You need to realize, he is with you now, if you trust him like you hopefully should, you need to understand he is with you for a reason. Jealousy is hard to get over, but let me ask you a question, do you sincerely trust your boyfriend? Do you feel he will leave you for her?