Please understand that what you see in a picture or video doesn't necessarily tell you the whole truth. Using silicones will make your hair appear healthy and shiny even when it isn't. Flat ironing frequently is going to damage your hair, and that's just the way it is. I'm not telling you not to straighten your hair, but you have to be realistic about it. Do it in the least damaging way possible. You may have to compromise on length, for instance, because the more you flat iron, the harder it will be to maintain decent ends beyond a certain length--and broken ends are very obvious on flat ironed hair. So in order to have both straight hair and nice ends, you may need to keep your hair somewhat shorter than you'd prefer. Life's all about compromise, though, and if you want your hair straight maybe you're willing to make that compromise.
Elumen is a good demi permanent. It can be a little drying, but not bad with a moisture treatment after, and it sticks very well. I don't know which drugstore brands are good, but Feria doesn't seem to have a very good reputation. And yes, start conditioning. It will really help with the dryness and make your hair feel better. It won't cure the damage--nothing short of cutting will help that--but appearance and feel will be better. The Tresemme Naturals Moisture conditioner (not the shampoo!) is cheap and good. My favorite of the conventional brands, although it's actually the only Tresemme product I like.
You might also want to check out the Longhair Community. Longhairs are really careful about protecting their hair, and you might get some good ideas. There's even people there with long hair (I mean waist +) who periodically straighten, although not often because frequent flat ironing just isn't compatible with really long hair. Discussion Forums
Former longhair, now CG, living in N. California where the fog meets the wind
3a/b, fine, normal porosity/elasticity, high density
Conditioner wash and style. That's it!