Young Male, digestive issues. In need of some answers

For nearly two years now, I haven't been able to get my BMs under control. Im 21 years old, and before feeling like I wasn't able to correctly have a BM I never thought twice about going poop and was completely normal.

So, I constantly feel like i have to poop/pass gas, I mean ALWAYS. Occasionally on the toilet I can have a decent BM (usually in mornings after waking up), but i never quite feel as if I empty out all the way so immediately after standing up I feel like I have to go again. And when I do actually have success on the toilet, its not solid whatsoever but almost like powder or flakey. And I can only poop in certain positions of sitting (the most success from holding my weight up with my arms, right before defecating). Over the past few month I have gotten better at just relaxing the sphincter on the toilet rather than trying to squeeze it out.
So I feel like my bowels don't have a problem moving, the problem comes when its actually time to release the feces, and I wonder if its from holding it in when I didn't want to go in public growing up. Almost like, my muscles in my sphincter - feels internal - wont relax enough to pass stool.

I had a Colonoscopy done back in July of 2014, a barium enema later that year, and have also tried biofeedback.
The GI doc found a polyp during the colonoscopy which he removed, nothing was found during the barium enema and the biofeedback did just not really seem to help.
The doctors said I could have a low transit digestive system or just plain constipated, so they just prescribed all different types of laxatives, miralax and others that I cant remember. One that increased fluids in the digestive tract and I literally crapped water that whole next morning.
I'm currently not any laxatives but am taking a probiotic which seems to help, but does seem to cause some nausea, btw.

The amount of feces piled up inside feels like its been wrecking my body, my lower back is super tight, and neck as stiff as a door. And I also get muscle spasms in the arch of my left foot, and occasionally (but not in a few months) in my left tricep. And I even get a lot of weird throbbing pain in my testicles occasionally which is kind of freaky.

I generally eat pretty healthy:
- lots of fruit
- vegetables, i blend a juice first thing every morning.
- not a lot of meat, chicken and beef occasionally.
- Tuna, rice, and beans for protein and sometimes a KIND bar.
- I stay away from bread, even though it doesnt seem to cause me problems (don't have celiacs)
- Never really eat processed or frozen foods

Basically I generally eat healthy, and have went through a lot of dietary changes which none have seemed to change anything.
I just need some relief, some solid answer or diagnosis from a doctor, so I can at least know where to begin. Or even some help from someone on here who has seen or experienced symptoms like these themselves, please really could use some help. thanks

Feces build up can get toxic over time and is generally not healthy. It can make you feel horrible. Have you tried eating smaller meals thru out the day to help ease the slow transit time if that is what is causing you issues?

A simple cause of this is your high fiber diet. This is how I was before my disease worsened into inflammatory bowel disease. I would reduce your fiber pronto and up your intake of animal based foods so your digestive system isn't having to work so hard to break down fiber.