Get tranexamic acid and/or northisterone from your GP first, as it will massively help to reduce heavy bleeding and control it, make sure you take it consistently so 2 every 6ish hours 3 times a day. Try tranexamic acid first.
I would perhaps try the mini-pill or the mirena coil over a combined pill, because often with the combined you take a week of and the breakthrough bleeding isn't helpful or pleasant. There are combined pills you don't have to do this for but there are less. Plus the mini-pill is progestogen only which is better for thining the lining and stopping heavy bleeding. With the mini pill you take it continuously, so no breakthrough bleeding.
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 - oestrogen does this), let's not go overboard and progesterone maintains the uterus lining (whereas after your period, estrogen stimulates it to grow more). hypothalamus, or pituitary glands at that point stop producing FSH, LH - that means that less estrogen is produced (LH and FSH's release triggerss release of estrogen).So more progesterone means that we can suppress estrogen secretion and thin the uterus lining a lot more than the mini pill - especially because it is localised into the uterus (helpful for periods, as it can thin it so less bleeding and cramps). So mirena mimics the luteal phase, in a more continuous dose. That's why it helps with symptoms.
For insertion there are things they can do, make sure you ask for this, and make sure you tell them in advance so it's ready for you. Perhaps call and ask the receptionist to put a note on the system. Get licodaine injected into the cervix, numbing gel, and take paracetamol and ibuprofen 1h before as well.
Also ask your GP for mefenamic acid and/or naproxen (NSAIDs which help a lot as they are anti-inflammatories and give pain relief). Start taking it 3/4 days before and it will be much better as it works at best capacity then, so take it 3/4 days before your period should start.
Also, I recommend going to asking your GP ask for a pelvic ultrasound, and a transvaginal one to see what they find.
Hopefully this makes sense, but if you have any questions just ask.