I'm sorry you are suffering so much. Can I ask why you feel like you need to suffer pain? I guess that's kind of personal though- no need to respond if you don't feel comfortable.
Honestly, I kind of struggle with these questions. Maybe there are knife experts on here but the only way someone is going to be able to answer this question absolutely is if they have killed someone else with a knife- in which case- the knife was strong enough or they tried using one on themselves and failed (because we are all still alive on here). I'm guessing it doesn't only depend on the knife though... It would depend on your own strength, resilience to pain- if the first cut doesn't work and you need to make more and your aim- to hit something crucial.
I do understand- we all want reassurance that our chosen method will work but sadly, it's hard to honestly give 100% reassurance for ANY method. The American Association of Suicidology estimates there are 25 attempts for every suicide completion. Sadly, it's not an easy thing to do.
You may be successful in your attempt but you may also severe major nerve endings and end up with long term health issues which prevent you from making any further attemps.
I'm sorry if it sounds like I'm just trampling over your method and feelings. It's actually coming from a place of concern. While I believe everyone has the right to take their own life, I think as a community, we should also warn each other of the potential failure/long term health risks of certain methods.
I do wish you all the best- whatever you decide on.