“Constipation Help”

I’m 17 years old and I’ve been struggling with constipation for as long as I can remember. I rarely ever have a normal, smooth poop, it’s usually pebble-like or like pebbles all stuck together. I’m not the most hydrated person in the world and I have a really bad diet, I am very picky and I only eat a select group of things because I know that I like them and I will eat that stuff, but I’m willing to try to eat certain things more often if I know that they’ll help with my constipation in the long run. But that’s why I’m asking a doctor if there’s any sort of vitamins, that would help me or if it’s just as simple as needing more water or what types of foods really help. A big issue I run into is where I’ll eat the normal food I like and some fast food is thrown into that, which makes my stomach hurt on its own, and poop just builds up and doesn’t come out for days because I’m that constipated and when it does come out, I get bad cramps and gas. I’ve never seen any blood though, so that’s a good thing. There’s no pain around my anus ever and when I’m pooping, but it’s just mainly like abdominal pain when I have to poop and when there’s a buildup of poop. Like bloating. The point of this is really just to know what I can do to help it and if there is some sort of underlying health issue like IBS which is something I’ve heard of. I’m getting blood work done in about a week because I just need to so maybe I can see somehow what’s wrong through that? I hope you can help me and get back to me. Thank you.