In my opinion, you didn't make a "mistake" - you were taking doctor's recommendations to try to ease the pain you were experiencing. Sadly, the first method for "controlling" period symptoms (at least in Western medicine) is birth control. Thousands of us jump on that bandwagon, hoping to relieve the pain.
The big problem? For many of us, the birth control seems to help a bit, but in reality it is actually exacerbating the condition, especially over the long term.
After years of doctor's visits, trips to the ER, and countless tests and ultrasounds (transvag ultrasounds are awful enough - much less when you are in several pain already!), I finally figured out what was causing my insanely heavy and painful periods. Endometriosis. But, at least for me, the endometriosis can be controlled with diet (I was severely allergic to a certain food and it was causing the endometriosis to flare) and exercise.
And my one concern with starting T? That all that pain and the heavy periods would come back (after two years of little cramping and normal blood flow). But, thankfully, that has not been the case! I am three months in and my period has completely ceased. Even the one period I had on T was short and painless.
Long story short: An excess of estrogen is often to blame for heavy and painful periods. Unfortunately, Western medicine tries to treat these symptoms with more estrogen (and/or progesterone). If excess estrogen IS to blame, taking T ends up being a fantastic treatment.
While you are waiting for the T to work its magic redistributing body fat (which started fairly quickly for me... ~1 month in), you can take steps to help your body eliminate the estrogen. Avoid things that stress the liver (alcohol, cigarettes, excessively fatty foods, etc), get lots of exercise, drink lots of water, and (if you can afford it or have someone who will do it for free) get massages. Also, losing weight helps overall. Estrogen is stored in the fat - less fat, less to store the estrogen!
Feel free to PM me if you want to discuss details further. After battling to find the information for myself, I am always happy to let people know what I've learned!