I can really relate to what you are saying. When you are not in shape, exercise doesn't feel that great! I think our bodies are a little sneaky and they complain alot when you first start working out. On top of that our brain tries to tell us that we are too tired, don't want to etc!
You are not a failure! Exercise is just like any new thing. When it is not a habit and you have not yet experienced its rewards- creating a new habit is hard!
The key is just to keep getting back on the horse. Failure would be to say I am never going to exercise, so get off my back- and you are not doing that!
I work as a personal trainer with alot of people in your boat- Heck, I became a trainer because I built your boat! My experience is that if you start to be regularly active at a low intensity at first- you will be more likely to succeed. The regular HABIT is more important than worrying about how hard you are working. That can come later. Still, I think that the first 3 weeks are the hardest. You will feel more tired, not more energetic. After that.... I think it gets easier.
Keep pluggin. You can do it! Think of everything you have already accomplished.
KatW