It's great that you are trying to assert yourself in your relationships. It's not always easy to stand up for yourself when you might be hearing messages that you shouldn't.
Keep practicing. The more you talk to your partner, the easier it becomes. Read about assertive communication and look for ways to improve.
Maintain a support system. Identify a friend you can talk to or make an appointment with student counselling.
Take self-responsibility for your actions and believe that you are doing the right thing by protecting yourself.
Know that the response from your partner will not always be what you want or expect. Identify possible difficulties or obstacles before they occur and develop a plan to overcome them.