I am very concerned about your post!
First, please try to calm down. Nothing ever stays the same.
Good and bad things happen, but that's life.
Never ever think that suicide is a way out, because things get better. Honestly!
Don't consider it ~ and if the idea rears its head ~ please get immediate help from a counsellor, or other reliable, trusted person.
The fact that you have even mentioned such an idea tells me that you need to mention this to your school counellor, anyway.
You say that the counsellor hasn't helped, but have you told him / her that suicide had even been on your mind?
You need to talk to them about this, get it off your chest & get some external thoughts on the idea.
You are 15 ~ and I get the impression that you are sensitive. I've been there! I used to suffer from stress, anxiety & depression throughout my teens. And meeting my boyfriend ~ now my husband ~ seemed to be the one thing that calmed me down!
It's down to all sorts of things ~ probably mainly hormones, but other things too. Other people seem to control your life. You are too old to be a child, but too young to be an adult.
Your parents may have worries, etc, etc.
And, of course, you must be worried about your Mum.
As for your parents, if your Mum is ill, with no hope of a cure, then she is likely to be feeling frightened, upset, angry and possibly even more stressed than you are. This in turn will make your Dad feel stressed.
They are probably worried about each other and about you. Maybe they don't know how to deal with this or how to express their feelings. Remember, adults are only grown up children. There is no reason why your parents shouldn't be feeling just as confused, stressed, upset, etc, as you are.
If your Dad is worried about your Mum's health, then he is bound to become angry if you shout at her. On the other hand, he does seem to understand, because you say that he sticks up for you.
What has now brought this to a head is the fact that you walked into your girlfriend's room while she was changing and you are worried that she ~ the one part of your life that brings you joy ~ will break up with you.
So you need to deal with this.
You need to apologise for what happened and reassure her that it was a mistake and that you had no idea that she was changing; that you thought that it was ok to come in, because her Mum sent you up, but that you realise that you should have knocked first and you are really sorry.
In fact, you should always knock, regardless ~ you have learned that now.
Get the message to her as quickly as you can. A letter would be good ~ but a text would be faster.
Is it possible, when your Dad is in a relaxed frame of mind, that you could talk to him ~ man to man.
Show him that you appreciate that he has worries & responsibilities, and that he is worried about your Mum. Explain that you are feeling depressed and worried & that things are getting to you & you are concerned that your grades are falling because of this.
Let him understand that you don't mean to upset your Mum, but that you are feeling very down, yourself, and that you need help to cope with it.
Do you fully understand your Mum's health problems?
Can she be helped?
Are things made worse or better by anything?
Is a quiet, personal chat with her a possibilty?
Make sure that you talk to your counsellor and tell him /her everything!
They cannot help if they don't know.
Take care of yourself!
Try to be positive!
You sound like a very nice lad, who has a great future ahead ~ just try to be a little patient.