You then have to decide that you are going to do it anyway, thus going against your conscience. The Catholic church officially teaches that the conscience of an individual is supreme. If you carefully examine your conscience and then decide that an abortion is the most moral act you can do at this time, you are not committing a sin. Therefore, you are not excommunicated. Nor need you tell it in confession since, in your case, abortion is not a sin.
Decisions involving procreative choice are serious decisions.
Know there is no one acceptable morally justifiable way of living your life. Know what you are deciding. No alternative will be all good or all bad. Conversations with trusted friends or unbiased counselors are good for anyone who faces a major choice. Clinics where abortions are performed can usually refer you to clergy or counselors who will raise the questions you should be asking.