It’s frustrating that people don’t accept that asexuality is real. It may help you to have some links and evidence to hand that will explain asexuality and prove it’s real. I posted these starting points in another answer, but I’ll repost them here because they’re useful when explaining or defending asexuality:
It sucks, but oftentimes sources like this can be more persuasive than individual accounts. If people continue to dismiss your asexuality, tell them that they’re being hurtful, disrespectful, and wilfully ignorant in the face of your experience and overwhelming evidence that asexuality exists.
Best of luck; let me know if I can help out it any other way.