said to her "by the way, I find you not bad" (maybe this sounds a bit awkward in English).

Yes, it does. Were you two speaking English to each other? Or did you translate something that makes total sense in your language, but not so much in English?

If you two were speaking English, but you have an adorable foreign accent, she might be forgiving of that odd turn of phrase.

Anyway, I'm getting too bogged down in something likely inconsequential.

How would you say this went, what do I need to adjust about my approach?

It's hard to say based only on text.

What a lot of people new to this sort of thing (or just rusty) is that a lot of the calibrating will be on your own, from a lot of trial and error.

me hesitating too much.

That's most likely it. I had a similar problem back when I was first starting out as a burgeoning young player-to-be.

Boldness is always better than hesitation.

If she's going to reject you anyway, you find out sooner and stop wasting your time and effort on someone just soaking up the attention.

If she's receptive, hesitation can kill her receptiveness (if that's even a word).

but then she brought up her plans for the weekend (some stuff with her friends) and that threw me off.

"Aww that's a shame, we could have gone and done ". The trick here is not to immediately agree to any rescheduling if she's interested.... it's a throwaway line and you don't immediately convert it into scheduling hell as she tries to line up both options.

not, but maybe" (here I messed up again, should have just asked her to do something off-campus but I was in her frame).

nah..... if she's busy for the next day or two, you don't push it. Stay aloof. No trying to agree for "three weeks on Tuesday if you find yourself free"

Then I mentioned I'll be there tomorrow for sure due to some stuff I have to work on, stood up to leave,

Ok, agreeing to be around tomorrow sucks. You are killing your mystery at this point.

and I smiled and said to her "by the way, I find you not bad" (maybe this sounds a bit awkward in English). She said something and laughed and I left saying "see you tomorrow, maybe".

Well, not sure if that's a good line or not. But "see you tomorrow maybe" is terrible - you're just committing yourself and she isn't doing the same. It sounds like you're too attached.

I feel a bit bad about how it went, when I cold approached her last year I was more cocky and there was some tension, and I had strong frame with her the whole year in between (I was in a LTR and didn't care about her)

Yeah, IDGAF is easy when you really don't give a fuck. It's harder when you're single.

which I think I have now lost. On the other hand I feel like I established a bit of frame

not much frame tbh.

and tension with the compliment at the end.

No tension. Compliment is cute, but all your other comments are showing your desire to attachment, commitment to an outcome, etc. This kills attraction because you're getting towards being a sure thing. You need to maintain distance, control, ambiguity, options. Or you date and fuck her. Avoid the bit in between because it costs you everything, gets you nothing, and kills attraction / fear of loss.

I later found out she is in a LTR, at least she didn't mention that to me.

They never do my dude...... they never do. Not until you push for a date. Not even then.

How would you say this went, what do I need to adjust about my approach? I have a few other prospects where I'm worried it will end up the same way - friendly conversations but no tingles and me hesitating too much.

Ok, don't worry about it. You want to have a LOAD of these interactions.

It's campus, so you can't escalate too hard.

I'd suggest: Smiles. Find excuses to touch her. Talk about plans and the cool stuff you've done and/or are doing next. Be aloof if you're not actually going on a date. No more "see you around". No "hope to see you soon please maybe". You want to be mysterious, distant, UNATTACHED. You interact and then you say "bye", not even "see ya" (which sub communicated a desire to see her again).

When you're there you're smiles and touching and friendly and maybe escalating to seeing her again, but other than that do not kill your mystery with attempts to get future contact with her.

Attempt to answer own question: I would say I need to be more polarising and cocky to give her tingles instead of just boring everyday small talk. Asking her if I can see her on campus tomorrow was bad for my frame.

Polarising and cocky can work, maybe this is your vibe, maybe not, I don't know. If 80% of women hate you and 20% are fucking you then you're doing it right.

I don't think any harm was done. She wasn't too keen sounding and you never really made a move. Then its ends up she has a boyfriend anyway. You could have gone charging in and asked her out but the result would have been the same -she wasn't up for it. Sometimes we can work things out without needing to ask.

Another time she might be keen, especially if she has split up with the boyfriend. Sure to some extent she will match your energy, so if you are upbeat and confident, she will have confidence in you but it also never hurts to read her energy. If she is not keen, then a pleasant chat is good. There might be a next time.

"Not bad" depends on your language, In French for example its not a bad thing at all to say. Its a bit understated but its heaps better than "I think you are a awesome" (and I will be a massive simp). There is a lot to say for being understated. Women always want big compliments -so let them work for them. Why not let her know you like her and also give her the idea that you know there is more out there that either she or others could bring? An understated gent is not bad at all.