How to determine insulin dose

How do T2’s determine how much insulin to take before a meal. I struggle with this at every meal. My Endo has me on a set amount but occasionally it doesn’t cover for all the carbs I consume or I go hypo because I did not consume enough carbs for the insulin dose.

I need to be more insulin savy. Last night before bed I dropped from 8.3 to 5.1 in 20min so I skipped the overnight Lantus. Around 4 am I took some glucose tablets becuse my BG was at 4.0. Today my BG is at 10.0 because I did not take the Lantus. So do I take a small dose of Lantus or up the novo rapid dose at meal times.

Sorry to be such a pain but I’m embarrassed that I cannot get control of the insulin doses.