Can you be more specific about your symptoms and what the investigations you had, have found so far? It may be till now you seem to have asymptomatic (from what you've said as you have only had the pain for a some months and not persisently, it may be late onset but also look into other conditions like adenomyosis as well - thee MRI should catch that). Which is good as the pill and NSAIDs should resolve it long-term.
Have you had chronic issues with mobility, back and sciatica, debilitating pelvic pain, pain with sex and insertion to the vagina, severe vomiting and nauesa, cycles of severe constipation and/or diarrhoea. Just trying to see about symptoms like what sort you have, it's very hard to diagnose diagrammatic endometriosis as it could also be a respiratory condition, so definitely see a respiratory doctor as well to check for even having another condition as obviously its a very important area. It doesn't completely sound like endometriosis to be honest, but have you got other symptoms or have you had ultrasounds etc that you haven't mentioned as I'd hate for a respiratory condition to be missed so definitely see respiratory as well.
I recommend asking the GP to try the pill and/ or mirena coil can be amazing with heavy bleeding and pain. You might try different pills first. Yes, they treat the real cause as well (mirena will make uterus lining thinner, less bleeding and less cramps) So during a menstrual cycle, progesterone is released later on (luteal phase) - it basically tells the hypothalamus and pituitary glands, stop now you've done your job ( which is thinking the uterus lining), let's not go overboard and progesterone maintains the uterus lining (whereas after your period, estrogen stimulates it to grow more). Essentially the hypothalamus, or pituitary glands at that point stop producing FSH, LH - that means that less estrogen is produced. So more progesterone means that we can suppress estrogen secretion and hopefully stop more growth of the uterus lining (helpful for periods, as it can thin it so less bleeding and cramps). It can also help to stop the growth of endo lesions elsewhere because endometriosis lesions can create their own oestrogen.
Also ask your GP for mefenamic acid and/or naproxen (NSAIDs which help a lot) and tranexamic acid (helps reduce heavy bleeding) - you need to start taking it days before your period starts so that it can work at best capacity. The same with ibuprofen and/or paracetamol. Start taking it 3/4 days before and it will be much better.
However, the only way for definite diagnosis is a laparoscopy and then they will often excise the endometriosis tissue if they find it etc. But endometriosis is a chronic illness and cannot be cured, after surgery it will grow back, it cannot be excised from everywhere to leave organ functioning. The main symptom of endometriosis is not actually period pain because endo is not a period condition - it's a whole body inflammatory condition where the endometriosis tissue even produces its own oestrogen and the pain is felt throughout the month not just when on your period. Especially if you are having breathing difficulties you'll need a different type of surgery, well to explore in that area - you'll need cardio throatic surgery to be there and do that part.
Mangagement can look like pain medication depending on how severe your pain and symptoms are (can be opioids, NSAIDs) and hormonal treatments.
The links below have much more detailed and useful information.
https://www.leedsth.nhs.uk/patients/resources/endometriosis-2/
https://www.nhs.uk/conditions/adenomyosis/