You need to be realistic. When you are in a downward spiral where the main symptom is extreme physical tiredness brought on by the relentless focus on work, the answer cannot be to work weekends on your own business idea. That is like suggesting an ME sufferer simply exercise their way out of their lack of energy!
Here are the facts.
1) If you want to make it in your current firm you buckle down, put everything else in your life on hold and “death march” your way to the end of the project. Many people do get better at handling the workload in time, and in some cases people discover afterwards that they were being tested by their firm to find out what they were made of.
2) If you feel this is not a temporary situation, nor one you are able/want to get used to then write “Self Awareness Day” in whatever you use as a diary and force yourself to work in sensible steps towards your goal. Spend absolutely no more energy on progressing your career at your current firm – you have no future there.
3) All smart people who launch their own business do so while being paid as an employee of another firm. All start up CEO’s I know (and I know dozens) all say they wish they could have kept themselves employed just a little longer while they did the groundwork. The impact on personal cashflow can be huge and that can make a big positive impact on the freedom you have to chase the right sort of business, employ the right sort off people, etc.
4) HOWEVER – you, can’t do all of this at a job that is killing you. SO, obvious answer is to take a temporary and small step back to a less well paid but less time consuming job.
5) Be aware that many people on taking that step back find they never step forward again! It turns out that the idea of starting their own firm was just that – an idea.