LYING! I got caught lying but for a good reason. Is it ok? Need opinions!

Let me start off by saying I'm condoning lying ever! That being said sometimes you are put in a position that lying may be the best option and only option. Long story short: I have medical issues that require me to take pain medication. I've been taking this medication for 5 years , 2 years before I met my girlfriend. When I met my girlfriend I told her I took this medication and she flipped out on me thinking that I was a drug addict. I told her that I wouldn't take it anymore because at the time it was a choice of the mess or her. Well the obvious is that I didn't stop taking them and about 6 months later she caught me taking them. She asked me if I was taking them and I told her no. She found a receipt from the dr. Visit and knew I was them but she just wanted to see if I would lie. Well she caught me in a lie and flipped out because I lied to her face. she said she would've gotten mad but not as mad if I told her the truth. I didn't believe that so that's why I lied. After that I told her I really would stop. Well I didn't and a week ago she caught me with one pill and flipped out beyond f and broke up me because she said I lied again. The only reason I'm lying about this is because I know how she feels about the meds but I have to take them! She doesn't understand this, never will and it will be an ongoing issue! Am I wrong for lying thinking I wouldn't get caught and choosing the path of least resistance.. Or is she wrong for originally the ultimatum on me and not understanding my medical condition and not understanding why I lied?.. I need an outside opinion desperately!

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