firstly, thank you for posting. It can be hard to be open and write what you are feeling into a space not knowing how people might reply.
I don't have the timeline issue you referred to... However after highschool I did not get in Uni. I went to TAFE instead and then Uni. Another person I went to school with repeated year 12 before going to Uni.
Now I don't know what you are studying at Uni or where you thoughts come from but there might be and also some positives in your post. For example, you might be learning stuff at Uni that you can apply at work and vice versa. You are also able to juggle work and study. Also gaining life skills.
On the abusive customers... I know from personal experience that it seems like they are talking directly to you and that it is all your fault. My psych also told me about reframing my thoughts. There may be another reason for the person being abusive. They may be running late, or had a bad day. Or? So they may be projecting their anger or frustration out on you. Even if you try this, it might not work first time but with practice and time ... At the end of the day you are doing the best that you can, and that is all you can do.
I could post on some of the other topics. I would suggest that you might consider talking to someone about these thoughts and feelings. I started seeing a psychologist about 3 years ago and still refer to myself as a work in progress. It might help you also? or myself, nothing was worse than holding all the pain and negative thoughts inside of me.
I hope you come back to share more of your story.