I bet money that your back does not look that bad. Sometimes mothers can make a big deal out of relatively small things and people in general can say things without thinking. I had a similar situation with my son who is about your age. He had chickenpox at age 7 and he ended up with a small pockmark on his cheek by his mouth. It was a nonissue....it was small and didn't look bad at all.
About a year ago, my wife and I went up to visit his sister at college. Since his place is near there, we dropped by unexpectedly to visit. When he came to the door, the side of his face was bright red and looked raw! Of course, we asked him what happened and he said it was very embarassing and that he would tell us later. He finally told me that he had had some procedure on his face at the dermatologist for his scar and it looks red like that when it is healing. He said that he was always really self conscious of his pockmark and felt that his face was ugly and unattractive because of it.

I was surprised because it never seemed to bother him. He said that when he was recovering from chickenpox, his aunt and mom were talking and his aunt asked his mother if he got any scarring. His mother said "oh yes, and it's right on his face...it's a shame because he's so handsome". And every since then he has felt that his face was no longer handsome.
I bring this story up to show that children internalize things and it is important to be mindful of what you say and not make a mountain out of a molehill so they won't develop a complex about things. I just told him that when scars are new, they look worse and after he fully recovered from chickepox, it didn't look bad at all. The mark was small and didn't make him ugly at all. He was also a bit embarassed because he had been caught getting something cosmetic done which is not considered "masqueline". I told him that it was no big deal....everyone cares about their appearance to a certain degree.
Later on, my wife asked me if he'd ever told me why his skin was so red. I told her what he'd told me and she was shocked and upset that something she'd said had made him feel bad about himself for all those years. She talked to him about it and he was fine. He did what the derm told him and his face healed and you cannot even see where the pockmark was unless you really really look closely.
Like I told my son, scars look worse when you first get them. They probably looked worse to your mom when you first got them. But over time, they mature and smooth out. You are not "ugly" and I am sure they don't look that bad. Your mother likely just said something without thinking and I'm sure she loves you very much and would be upset if she knew she had caused you to feel bad about your body. If you are concerned about the marks, you can talk to a dermatologist or plastic surgeon about them. But chickenpox are not like burns or surgical scars. I am sure that they don't look that bad and whomever you finally become intimate with will be lucky to have you! If you are insecure about your body, talk to the woman before anything sexual happens and just tell her that it is your first time and that you are insecure about your body. If she is worthy of you, she will try to make you feel more comfortable. Scars or no scars, everyone is nervous their first time. If you were not, you would not be human. The more experience you get, the more comfortable you will become. Good luck.