Inform this to your boyfriend, make a calm, polite and nice but with a serious manner discussion with him about it. Explain how you feel and ask him to try to understand you. Communicate with each other and find a mutual solution for this until your feelings get cured by time.
When my boyfriend goes out with his friends I get really sad. I know it's selfish of me, but I was cheated on in my first relationship, and I guess I'm insecure..I want to stop this, please help?
Be secure enough in who you are to let him go out with his friends. Find something else to do with your time besides moping at home, wishing he was there with you.
A well-rounded friendship / relationship gets better with time as people learn and grow. Keep yourself busy with your own friends, hobbies, volunteer work, a 2nd job, something else besides worrying about what he is doing.
He can sense your distrust and it could damage this relationship. You don't want your jealousy and insecurity to drive this train. When you find your own happiness, your relationship will be more fulfilling, not less.
An insecurity like that, although difficult to endure, is not impossible. It takes a tremendous amount of courage to trust again .. Trust is a double edged sword.
Not only do you have to make a conscious effort to dismantle your insecurities in order to trust again .. But he also must be trust 'worthy'. The only way to instil balance is for you to be convinced that he is 'worth' trusting. Then the future may have potential.