<human>: I haven't really learned to say no when I grew up. I come from a family with a physically abusive father and as a child I was not allowed to answer back, disagree or say no to him without fearing to get beaten.
I've seen a therapist for 4 years, attended self defense and martial arts classes and I feel a lot more confident than some years ago, however I still struggle to say no, especially to men who are older than me and are close to the age of my father.

There is this older guy "Peter" (about 60) in my swim club who used to be my swimming instructor. He is always telling me he wants to help me in becoming a better swimmer, he already offered to train with me after my training is over, to go on a lake together to practice free water swimming or to drive me to a swim meet.
I feel unconformatable in Peter's presence as he's always staring at my body when he's talking to me (and I only wear a swimsuit) and I've said to him several times that I'm not interested or have other people who I train with. I feel like he may be sexually interested in me (and I'm totally not in him), I wouldn't want to spend any time alone with him.
Peter stopped asking me for some months, but two days ago, he was there again and offered to help me again. What can I do to make him stop talking to me? 

(I'm not a native speaker of English, so my wording is probably a bit awkward.)

Condense the text into a few key points
<bot>: I have a problem declining my former swimming instructor's offers to help me/ meet me privately. Please give me some advice on how to say no.