How can I make my girlfriend understand why this has hurt my feelings?
Tell her what you told us.
I have an uneasy feeling about this particularly because for me (and so I assumed almost all women), sex and love are strongly connected such that I fear that she might not be as emotionally attracted to me as I had thought.
Then tell her you want to have a more honest/in-depth conversation about the topic, if that'll help you. But also make sure you understand why this has hurt your feelings.
It seems your hurt comes from the realization that you are not the best lover your partner has ever had, which does make you sound insecure. It also seems like you think that quality of sex is linked to depth of love, which it is not. Sex is a learned skill, and people who have a lot of experience often outperform those who don't have it. That's not really a problem though; she seems to love you enough to help you get that experience.
You're better off asking her for confirmation of that part. I've had a lover who had tons of experience, and she was very good at reaffirming that even though I'm not very experienced, she loved me deeply and that mattered way more to her than the better sex she had with some other guys.
It's also why I say you should make sure you understand where the hurt is coming from. It appears to be an insecurity on your end.
How can I tell her that I would prefer if she did not stay in touch with this friend without coming off as controlling?
You don't. Trying to tell a partner who they can or cannot have as a friend is a huge red flag in terms of controlling / toxic behavior and you probably don't want to go there.
Especially since the problem exists within you, not between the two of them. They don't seem to be doing anything wrong.
If you suspect something is up between them, you're better off just bringing that up in conversation. Ask her about the kind of (friendly) relationship she has with the other man. And then decide if you trust her, or if you don't. Trying to control people because you don't trust them is counterproductive.