I mean, the neck or heart would probably be the most effective. But if you go with the neck you'll likely slash not just the jugular or carotid arteries, but your trachea too. Which will cause you to be unable to breath and choke on your own blood. I have no idea what getting stabbed in the heart feels like, but it seems less brutal to me since you're not suffocating. But actually getting to it is much more difficult: rib cage, cartilage, muscle. There's a lot more in the way and you'll need something more piercing than slashing (and long enough too). So getting to the actually fatal part is a lot more difficult and more painful (but less suffering overall). You could also stabbed yourself in the stomach (think seppuku) and while easier to get through than the heart, you would have to cut open your whole gut and it is a terrible way to die.
Overall, CTB by stabbing is an incredibly painful way to go and I would not recommend it. Consider other methods such as SN since it's considered a peaceful method and if getting materials is a problem, rope is pretty easy to get for hanging and I'm sure there's a tall building (jump from at least 50 ft, but try to do it from 70 or above, and land on your neck, instant death) near you that you could jump off of if you have no other options.
I wish you peace. I plan to leave soon myself