Maybe you know my son. Ha. He has this same issue every single day. Not the fear of pregnancy thing but the constant worry and fear that something terrible is going to happen. He envisions everything as a catastrophe that has either happened or is about to happen. Simple things that most people let roll off their back, he just can't. I'll get a text saying "this is so horrible. I'm terrible. I won't get into college. On and on. And I'll ask what happened and he'll respond he missed a point on a Spanish quiz. It has to be exhausting for him. As I'm sure this is for you. So, I really feel for you.
I know with family that isn't supportive of therapy, it's hard. However, there are resources that you could try on your own. First, you are in high school. You have a counselor. They should have materials for you to borrow on the subject. They may have therapists affiliated with your school. Our high school does. And they talk to kids when directed to do so by the counselor. This is a hard age that you are at. You aren't alone with the issues of anxiety. And sometimes, having a counselor talk to your parents about what they see with you (IE; anxiety) can bridge the way to their acceptance. I do think Paxiled also makes a good point. As a mom myself, when I feel helpless when my son tells me something that upsets me FOR him, I can have a strong reaction. Because of my helplessness and fear deep inside. Sometimes what I portray isn't in line with the real cause of my reaction which is worry. Anyway, talking to your counselor may net some help. Second, they have work books for teens on anxiety. They are nicely written and easy to work with and full of strategies. Amazon or ebay has them with a price tag of about 10 to 14 dollars. Your local library probably has some books as well. Third, talk about it with people (us, here?). You're not alone.
You don't say how old you are, but I'm assuming you're pretty young. A few questions: how long have you been feeling this way (the anxiety)? Is it recent or have you felt like this for a long time. How old are you, because it does matter -- it tells us how much your hormones are raging, what you're going through, etc. Some ages are just hard and then it gets easier before it gets harder again etc. Life changes. I do think that before you start engaging in sexual activity, like anything else you do you should learn how it works. If you truly believe you can get pregnant that way, learn how it actually happens (it's not that way). If you're using this as an example of just how irrationally fearful you've become despite knowing full well you can't get pregnant that way, then that's an anxiety problem. Pills are not a great idea for young people, though sometimes they are necessary. By the way, if you take any medication and it makes you ill, there are usually others to try that might not do that. But be careful -- if you're taking these drugs regularly, they can't just be stopped, you have to taper off of them, so do know that. But the advice you got to see a therapist, given what you've said here, is the right way to go. It is hard sometimes to talk to parents about certain things, especially sexual things. As to them not believing you, maybe you're exaggerating or misunderstanding. Most young people think their parents aren't listening to them. Often, it's true, just as it's true you probably don't listen to them much of the time, either. That's how families work, and if you're a teenager, well, the teenage years get really weird between parents and kids quite often. If you're really that scared that often and it has been happening for a long period of time, a therapist is a very good idea. And if you sit your parents down and really explain it to them, you might be surprised -- they might just not be understanding how much this is growing in you. If your parents truly don't want to engage in this with you, that's harder, and you have to find other resources, such as asking counselors at school or talking to your primary care physician about where to go for help in your community. But start with your folks, be nice about it, and be earnest with them. Be patient.