Hey, today I went to the lake from this girl to my class for a quick swim. (She has a boyfriend).
We were walking there, and the conversation was simply factual and non-sexual, not even close. We got to the lake, and despite the little sexual tension coming from me being shirtless, and us jumping in the lake, I couldn't make it sexual.

We later decided to climb trees (yes, we climbed trees, ok?), and there was one point where we were really close to eachother. But because it would just be weird to do something and because I didn't sexually escalate before and because she has a boyfriend I didn't do anything. I knew that she could feel some sexual tension if I acted right, I mean we were going swimming to the lake, it was sunny and free! But I didn't, How do I do this wthout making it too obvious.
How do I switch tone also? How do you switch the slow of conversation?

How does one be more sexual and interesting? How do u switch from the boring convos to the interesting ones? This is my sticking point.

If you search you can find many ways to discreetly be more sexual. If I were you I would find any reason to get close to her, but don't do it creepily. Make a good excuse to get close to her. find excuses to touch her. Any bodily contact can be sexual. The key is to make sure you always keep eye contact. Don't ever be the first one to look away. Look into her eyes until she gets so nervous she looks away. You must be confident. I would read in the "inner game" section