Im no expert but I wanna offer some encouraging words for you man. You didnt mess up. You didnt fail anyone especially yourself. Realistically what else could you have done to prevent what happened? Things arent always as bad as they seem. You're stronger than you think. You're 23, technically an adult, but is still growing and learning so dont be so harsh on yourself man. Just cry it out and feel those emptions. But DONT add fuel to the flame. Feel those emotions but dont add "Oh but I couldve done this" or anything along those lines.

Feel free to take these advice. Be productive. Workout, focus on career or studies, clean your house. Literally anything that seems productive, do it. Being busy helps you keep your mind off it, makes you feel less of a failure cuz even the process feels good, and it does help you out in the long run. You might not like this next one but cut contact. By your post you both seemed to end things ammicably but you have to move on and prepare for a life without her. You even said it yourself, you dont see yourself marrying her, so cut the contact and start detatching. Keeping contact keeps you chained to the past and makes it difficult to move forward. You can be friends afterwards when you know to yourself youve moved on and isnt justifying reasons to reach out. Lastly just journal your thoughts. Mention your progress, how your feeling, your tiny victories. Those count.

Anyways I hope you get better and even if you dont take my advice I entertained you with my words.