@Bansaw - stick around, you're going to learn loads!
(Uh, not from this post which is, I believe, of ordinary quality, but from PR as a whole!)
I did what you were doing at first, too. And my limits were where yours are. Almost exactly, except my Point of No Return was about 20 minutes of activity. Fifteen I was okay with. I tried going forward and back, forward and back, doing something I did not know was GET or its cousin. No matter how many times I retreated to fifteen minutes of activity, when I started inching up again, I experienced the same symptoms at the same timeframe. I did this with three crashes over a period of two weeks before I admitted to myself I wasn't getting anywhere, and stopped.
For me, the key to doing better has been pacing myself. When I have extra energy, I do a bit more. When I run out of energy (often suddenly and seemingly inexplicably) I sit down or lie down. Immediately.
When you feel good, be (gently!) active, checking in on your body and what it's telling you
often. When you begin to feel crappy (read 'exhausted, overstimulated or both'), that's it - you've got to sit down, and it has to happen in the next minute or two. Really crappy? Bed.
Now. No, you can't 'just' do this or that one more thing that you find super-important in the moment. You have to stop when your body tells you to stop. For me, this was the hardest lesson. I'd be puttering about my apartment, baking, and be all "whoops - that's it" and have to put the dry ingredients in a Ziplock and go lie down. I've finally trained my pride to accept that this is the way it's gotta be. I've done the same with close friends, explaining that sometimes on an outing I will have to leave suddenly.
I quit my job that had me running around like a mad person and got a job that's online, where I can log in whenever I have the energy to do so, and can dictate to my phone if I have a really truly terrible day. I've admitted I'm an utter slave to my body's needs - my physical being owns me, moreso than it seems to for other people, and that's the way things are right now.
You may have adrenal issues as well as ME (or ME may be partially due to adrenal issues). Regardless you need to start looking at how to best look after yourself, and please stop trying to 'exercise yourself well'.
A note to those from other countries - in the US of A when we say adrenal insufficiency, we are referring to the entire HPA axis: problems at the hypo, pit, or adrenals. Don't ask me why, I didn't coin it!
I have a blog about my ME experience which includes OTC supplements that I use. Good luck!
-J