It is absolutely normal in your position to feel that your family would be better off without you, but you need to know it is the opposite of the truth.
You killing yourself would have a ripple affect on everyone around you, but particularly your son and partner.
You can't see it, because your self worth is so low at the moment, but losing you in this way would literally ruin both their lives. It is the cruelest blow to loved ones, to find out that they had not known or not been able to help you, and they will blame themselves for ever. Their own mental health will be damaged and more precarious for ever because of your actions.
That is not to say that people who kill themselves are being selfish, because their mental state is so distorted, they genuinely believe the world would be better without them, but it is important you know that your thinking is distorted and not true.
The good news is, your feeling of responsibility to your little boy is a really strong protective factor which is stopping you acting on your feelings. I suspect that's one reason why suicide rates for women are lower than those for men - many women may feel a stronger responsibility for those around them which acts a brake on that terrible urge.
As others have said, you 100% need to talk to a doctor. The ones who go through with it are the ones who are unable to ask for help and admit they feel they have failed. By posting here, you've shown you can ask for help, and that is a protective factor too.
Well done for facing up to what you are feeling and sharing it here. Now you need to take that next step and talk to your GP. It seems impossible now but things can get better and you will not feel like this for ever.