Hey there, First of all I think it is important to be true to yourself and honest with your mother. If you are not a Jehovah's witness, you should tell her. Fooling her is only lying and would be wrong. Plus it would further stress you to pretend to be something you are not; obviously at this point in time you don' t need any more stress. I know it's a scary though - telling your mother that you don't agree with her particular religion. You can do it, though. This will get her on the track of accepting you for who you are and not who you pretend to be.
On to more important issues; You claimed that you have suicidal thoughts. There is a different between having suicidal thoughts and being literally suicidal. Have you ever came very close to suicide? Have you been morbidly depressed for a long time? If so, I advise that you seek help immediately. In the long run, some good therapy is beneficial to you. Talk with a medical professional and they will really help you work through your emotions. If you know exactly where your anxiety is coming from then it will make things all the more easy to up-root and get rid of. If you really are to scared to talk to someone face to face, I advise calling a suicide hotline regularly. It is completely anonymous but gives you an outlet where a professional psychologist can speak to you. Yes, posting more on LiveWire would help in a way; but by calling a hotline you can assure that someone will take you 100% serious.
One thing I would like to mention to kind of give you inscentive to get help is the well-being of your brother and niece. You said that you are basically the guardian for them. It's only fair to them that you are emotionally balanced and not in danger of removing yourself from their world. They need you and they depend on you. That alone should be the only motivation you need to put yourself in an institution until you are better. It would be a lot better to remove yourself temporarily then to remove yourself permanently.
About your medication; even when you feel like you are doing better it is best to be on the safe side. You never know when things are going to get hectic again. If your medicine really helps you cope better with stress then it is important that you stick to the daily routine. Do it for your brother and niece. Do it so they don't have to live without you around. I'm sure that the medicine is hard to deal with; but it's better then the latter option.
Some things I would suggest to better cope with stress:
Rubber band method This is most commonly used for people who like to cut. However, it's also advised to people who deal with a high amount of stress daily. It will distract any self mutilating thoughts that you may have by isolating your thoughts to the snap of a rubber band against your wrist. It has been proven highly effective. Just wear a semi-tight rubber band on your wrist and snap it when you feel a rush of anxiety on the brink.
Time with friends Time with your friends can prove to be a good thorough distraction. Sometimes that's exactly what someone needs to get them through the week. Try to set apart some time to go out with friends or perhaps a weekly meeting for a club. If this isn't a possibility because you really can't get away from your brother and niece, you could try taking them to the movies or to the park. Anything to get you out of the house.
Family time By family time I meant specifically time with your brother and niece. Any time you feel a suicidal thought; go and play with these two. I'm sure that when you see them you will realize how much they really need you. Keep them with you as much as possible for self motivation.
If none of these are effective and you continue to have suicidal thoughts in the extreme; I really advise you to look for a good institution to stay at. It's better then death. I'm sure that your brother and your niece would much rather go a few months without you then their entire life.
Please get the proper help that you need.
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Antidisestablishmentarianism.