The mind is a delicate thing and everyone's mind is different. As a result, everyone's experience is different when it comes to meds. Like you, I haven't found a combination that lasts more than a few months at a time. I've lost track of the different meds, different combinations, and different dosages I have been on. So far I've been able to convince myself that it's worth trying new ones as the affect wears off. Don't give up, it can be a long process with many setbacks.
I wish I had an answer for you. My medication if I am lucky will last about six months and then I end up on a new combination that may last three months, four months, maybe six. I had a combo once that lasted several years and I was able to work and everything but alas, that ended as well and I am back on disability. All I know is that it is trial and error and sometimes the meds last and sometimes they don't I hope you can find one that does the trick for you for a while. Don't get discouraged you are not alone!!