It is common to be afraid and anxious when we have a medical issue. I am there with you, I have Health Anxiety. I know what it's like to invent diseases and scare myself into thinking I'm going to die.
The trick is to think logically, and I know how hard it is. But think about it, your dentist is a professional in his field, he's studied probably for the better part of a decade. He's seen countless patients over the years. Do you really think this is something new to him he hasn't experienced or learned about? Do you think if there was any risk of you dying or getting a disease from the tooth he would have waited until Friday to see you? The fact is what you have is common place, he deals with it regularly. He knows there is no risk or he would have you in there and and fixed up immediately.
I'm not sure where you get the idea of an abscess from. You mention it in your subject title, but in the post you only mention you need a root canal. So, it seems you have an illogical fear of getting an abscess or infection, you don't really have one. You need a root canal, I assume, because you have a severe cavity that needs a very large filling. The large filling would impinge upon the nerve of your tooth causing pain. So you need a root canal in order to remove the nerve so there will be no pain. You didn't have this pain before your dentist told you you needed a root canal. The pain is therefore most likely somatized anxiety. You are concentrating on your tooth so much that your mind is actually causing the pain. The more you concentrate on it the worse the pain will be. You may even continue to feel the pain after the dentist has performed the root canal. This could get you worried again. You might conjure up scary thoughts such as, what if he made a mistake.
I hope that by shedding light on this subject I have helped to calm you down. You are fine, and your tooth will be fine. If I had the chance to bet my life savings on it I would. There is always a sense of urgency with HA. I know I often feel like, I have to get to the hospital now, or something bad will happen. It never does. In fact, by going I'm only feeding that fear. People with anxiety feel like we always have to be in control, and that includes our health. But in the end, we are powerless. We are given a short amount of time on this earth. The best we can do is take good care of ourselves and enjoy that time. It's through this release and acceptance that anxiety can be cured. I'm learning how to do that myself. It's not easy, but we can do it.