First, it will be good to figure out which aspects of intimacy will make you tense and concerned. If you are not keen on pre-marital sex, you can learn to be assertive and say no to your boyfriend. Many women try to sustain the relationship by compromising too much, but that may not be a good idea for a long term relationship. Both couples need to have give and take in order for a relationship to work.
If you are worried about pregnancy, you can ask your boyfriend to use a condom or consider other forms of contraception. It will be good to seek advise from a family physician regarding contraception.
The two of you can also engage in foreplay longer so that you can have time to be aroused and be ready for the next stage. Some guys have premature ejaculation (that is, they come too early) and hence prefer to proceed to penetration very early so that they will not come outside the partner's body. If that is the case, your partner may also need evaluation and counselling from a doctor.
You can also learn various relaxation techniques to calm yourself down. You may want to consider discussing your concerns with a mental health professional who has experience in helping persons and couples with sexual concerns.
