Your question is complicated in that it requires two answers. If you want to know if diet pills decreased the effectiveness of birth control pills, there is not enough data in the medical scientific literature to say one way or the other. It simply has not been studied enough.
On the other hand, if you are asking if is okay to take diet pills without your doctors consent, the answer is no. Many over the counter diet pills can be dangerous to your health. For example, some diet products contain Ephedrine, which is an amphetamine like compound (stimulant) that has potential powerful effects on the central nervous system (CNS) and heart. Ephedrine can cause high blood pressure, irregularities in heart rate, insomnia, nervousness, tremors, headaches and seizures. Other diet pills have similar stimulants properties that can cause the same problems.
You need to be especially careful since you said that you have been experiencing many health problems. Talk to your doctor and explain your concerns. He/she will give you precise information how diet pills will affect your health. Your doctor can also recommend a safer ways for you to maintain a healthy weight.