I'm so sorry you are feeling so low but please now you aren't on your own. The effect endo has on us can be so horrid, it's enough to make even the most positive person feel incredibly low.
My first thought is that you should go to your GP and explain about your mood. They can help. It won't take away the problem but will help you deal with it all a little more.
Secondly, I have had and do have the fatigue you describe. I'm 31 now, only just diagnosed but had gynae probs since age of 22. Through my teens I suffered a lot with fatigue, exhaustion, which carried on and as I'm going through a bad endo patch at the mo it's back with a vengeance. There isn't much sadly you can do about it, other than go with your body and sleep. Again, talk to your GP about this - but it may be worth being signed off work for a bit. No one likes being off work but sometimes we need to.
How are they treating your endo? If there is a treatment plan in place over time that shd alleviate symptoms. If you don't have a treatment plan go back to your gynaecologist.
It's a miserable place to be, and you can just feel that you have no quality of life but it won't always be as hard. I can't promise it will disappear but you will come to manage it and treatment can really help.
With regard to the weight, the only thing I have done to tackle this is begin the endometriosis diet (google it). It is pretty strict, but not only helps with weight but also with symptons including pain and tiredness. Doing something like this can help you feel as though you are at least in control of one thing, when so much else is out if control.
Please know though that you aren't alone and the way you are feeling is entirely natural.
Do talk to your GP about it all, and if you can get support (even if its just a hug) from those close to you.
I hope things starts to improve at some point for you. God bless xx