I would ask the GP to refer you to a gynae asap. You need to know why this is happening along with a diagnosis.
Such problems are often caused by hormonal imbalances; in this regard a condition called polycystic ovarian syndrome (PCOS) is a common culprit. Blood tests can and should be done to see if this condition is present. Given that your periods have always been irregular as well PCOS cannot be ruled out here.
OPKs and temperature charting are not worth using at all with such long cycles. Save your money.
The blood tests too can be done according to calendar days; you need a day 2 test done to start with to check and compare your LH level against that of your FSH level. Would think these two hormone levels are well awry.
Would therefore suggest you seek medical advice asap, do not leave this to chance any longer. With such irregularity of periods it is likely that you are not ovulating regularly if at all currently.