Oh, Erika, he will always need you! Always and forever - don't you forget that.
DS1 almost finished my off, I am not kidding, and I was, oh almost 20 years older than you when I had him. Not that age as such comes into it, but I had had my carefree teens/twenties and thought I was well ready for a baby. It took us 5 years and 3 MCs to make him - and still, like you, there were days I just dispaired.
Please do not think that getting to the end of your tether and exasperated and thinking unkind thoughts about him make you a bad person or mother. Anybody who claims they have never had such thoughts about their children is lying. And yes, some of us take to motherhood more easily and some don't. Some babies are 'easier' and some more 'demanding' 'high need' or whatever you want to call it.
It is v sad that that dark place is the only place you know, and of course familiarity can feel comfortable, dangerously comfortable.. But that does not mean that that place is the only one you will ever know.
Do not waste anymore precious mental energy on feeling guilty either: throw away that stick you are beating yourself up with - every single mother on this planet lives with some degree of guilt and it is a waste of time.
IMO, life with children gets better as they get older. I love my conversations with my big boys now. And yes, sometimes I could positively strangle them - still (like today...).
Do you have any contact with mums your age? I know I've asked before - nevermind if you do not want to answer.
Have phone no for Samaritans next to your phone/in your mobile.
You ARE the centre of Colin's universe, whether you feed him yourself or not. That is a great responsibility and honour and everything else, I know.
IMO (I am not a psychiatrist - have you got one? Apart from your therapist, I mean) you could take 60mg of Fluoxetine daily and still BF. How about a CPN/HV/SW mental health team/mental health crisis team for support?
Pick up the phone and ask for help before you do more than superficial cuts...