A spongy uterus is a classic indicator of adenomyosis. It might be rare to be diagnosed with it at such an early age, but it is possible to have it that young.
It is very difficult to diagnose adenomyosis, so a whole bunch of other stuff has to be eliminated first before getting any real answers from a doctor. However, with the better resolution US and MRI machines it is getting easier to detect these days.
Like the OP, I'd had a lap, but no endo was found. And I don't fit the "typical" profile either- too young and never had a full-term pregancy. Welcome to the 20% club.
I hope your doctor believes you about the pain at least. It was a real battle to get mine to believe me and I suffered with horribly debilitating pain for years before she would help. At least now I get a little relief, but still far from ideal.
Unfortunately, hysterectomy is the only permanent cure for (diffuse) adenomyosis, but will not always help with endometriosis pain.
If you want to have kids someday, there is still some hope. You might have a hard time, but it isn't impossible. That's why the doc won't give you a hysterectomy yet.
I wish you well with your treatment, and hope you have a better time than I've had so far.