Usual preface of mobile user and English not being my first language.

I don’t want to go into too much detail because many of my family use reddit, so this first post will be quite short and to the point.

This started about 7 months ago. I follow many subreddits for times when I am looking for material to masturbate to. One of which is dedicated to older women, as it is a fetish of mine. One day whilst browsing one of these subs, I came across a picture of a woman, without her face showing, but I was immediately 90% sure this was my grandma. The furniture in the picture looked very familiar and my grandma has a pretty distinctive body shape. I don’t know what led me to comment, I think it was a mixture of curiosity and intrigue. I commented and as the post wasn’t very popular, she almost immediately replied via messages. We spoke for a little while and she was flirty throughout. She asked me many questions about myself and always showed sexual interest throughout the conversation. I was uncomfortable at first, but wanted to confirm whether this was my grandma most of all. Again, I don’t know why this. When she asked if I could add her on a separate messaging app, I thought it would be a good opportunity to try and find out. I also didn’t want to decline as that may have looked very suspicious and she would have ended the conversation there. Wee added each other and she sent some sexual pictures, without me asking for them. I was very horny at the time and as you couldn’t see her face in them, I was ashamed to say I used them to masturbate to. Since this time, we have often exchanged pictures/videos and mutual masturbation. I have not revealed my face, but I have however seen hers and can confirm that it is in fact my grandma. I am disgusted every time I finish but I cannot seem to stop messaging her. Now the problem and the reason I am writing the post today is because soon we will have a pool party with family (it is allowed where I am) and I am afraid she will recognize my body when I take off my clothes to swim. So my question is, what can I do and how can I stop this all?