Hello Friend, Good that you are seeking a help. Any relationship with last only if there is trust, open communication and match of each others expectations.
If you are convinced that he is not your choice, then accept that as right decision and move on. What you are experiencing is quite normal phenomenon for any brake up. Give some time to accept and adjust.
Take a counseling support to cop up and how to avoid any guilty or negative feelings about your decision. It's important to do this, otherwise you will not be able to accept any new relationships in the future.
Next Steps
Consult a psychologists directly or online. Feel free to reach out to me online through Practo app for further evaluation and suitable techniques to overcome the negative thoughts. You will be alright soon.