irregular periods

so I'm new to this app, and I have had irregular periods since like 13 when I first started, it came every 6 months at first but has changed recently to every 3 months. ive tried birth control, I've tried to seek medical help, but every time I've made an appointment to see a gynecologist they won't do a check because I'm not 21, I even had one tell me I had irritable bowel syndrome (Which i don't at all). I'm worried I won't be able to have children in the future because of this issue... if anyone could give me advice about this I'd really appreciate it..