Hi there,
I'm not a birth mother, but I'm at an interesting place in the adoption triad since I know birth mothers (and women who almost became birth mothers but kept custory), several of my close friends were adopted and I'm an adoptive mother. I've seen quite a bit from all sides when it comes to adoption, which has been interesting.
Mothers are lionized by society, particularly birth mothers. Obviously maternal separation has consequences, but sometimes they're way preferable than the alternative and it can be beneficial to separate mothers from their children.
Bringing another perspective to the table, someone from my biological family who I adore had an absolutely awful (biological) mother. She was not abusive in the physical sense, but was cold and resentful of him from birth and never cared about him or his emotional needs. He noticed (the whole family did, as a matter of fact) and they just quietly hated each other until the day she died. No special mummy bond, no reconciliation and no change to what was just plain lifelong hatred. As a result of this, he had lifelong struggles with recognizing his own emotions and needs, as well as expressing them. I bring this up because this is a reality that can be true too but few people speak of.
Relationships are complicated, and giving birth or adopting doesn't give you a special bond with a baby if you don't, well, bond. I can't see the future, so I don't know if you will bond with your son or not, but since you speak of wanting to not be here any longer if he remains in your care, it doesn't sound like you have so far. Motherhood is in reality a relationship like any other, it can come to an end and sometimes it does so terribly. The book "Regretting motherhood" by Orna Donath might help you find perspectives of women who have been in similar situations.
You don't sound like a selfish monster; on the contrary, you sound intelligent, and there's plenty of women who regret motherhood. You obviously are mature enough to voice your feelings and aware enough to know you're not in a good place, unsure if you can provide your son with what he needs. That puts you miles ahead of other regretful mothers. Trying to do what's best for your child and your mental health is tough, and you have my respect for trying to find a solution, even if it involves some degree of separation and is not what's socially accepted. Since there's no danger, having regular contact is a great idea and should be in the cards anyways, should you decide for adoption.
Respectfully, your mother should respect your choice, especially if she's not in a position to take care of the baby herself. Try to look into postnatal depression and see if that might be affecting you. If you decide to place your son for adoption, Social services will first try a kinship placement before moving on to traditional adoption, so if there's someone suitable in your family you might reach out to them for help.
Best of luck, whichever you decide.