I think you might be struggling with picking your 'target audience' for your practices. You said you would like people to 'just have a conversation with you'. But if there's a language barrier involved, a conversation isn't something you 'just' have, it becomes something that actually takes a lot of effort. Let me give you a few examples:
I've worked retail, and one of the things that's in my top 10 of annoying things are people that try to have conversations that hold up the line. If you add a very clear language barrier on top of that, I can confirm I'd be one of these cashiers that will probably switch and try whether speaking English (or, if I have some knowledge of your native language, that language) to you will move the line along quicker/answer your questions faster/be understood quicker.
Teachers are also not so great for practising your Dutch skills: They probably already know you're a foreign student, and that your English is much better. I've seen teachers (whose native language wasn't Dutch) use English in these cases, not necessarily because they're busy but because they know that you'll understand them better when they talk English: It's about them feeling more confident their message comes across, not having to explain things multiple times, and avoiding misunderstandings due to miscommunications.
People you're trying to become friends with, or whom you're already vaguely friendly with, will probably also default to English for the same reasons a teacher might: It makes them feel more confident they'll be understood, and that they understand you correctly. It removes the risk of misunderstandings and avoids a whole lot of 'work' from just having a conversation with someone.
You said that so far you've asked one person whether she'd be willing to practice with you, and she wasn't very enthusiastic about it, at least not until your Dutch was a bit more fluent. It might be yet another sign that perhaps having 'just a conversation' with you isn't as easy as you might think for the people that have to listen to you.
My best advice would be to understand that people usually try to avoid misunderstandings in communicating with you, and to keep trying and find people that would be willing to practice your Dutch with you. Do not give up, keep asking people.
Given the amount of effort that might be involved in having these conversations with you, try and find people that you're growing closer with, that you made friends with. Friends are usually more willing to provide you with their time and effort than strangers, at least mine do.
People with who you have a closer relationship also often might understand you better, even if you do mess up your words. Good friends or siblings can sometimes be said to 'finish each other's sentences' after all. It might thus be worth it to first build a solid foundation in English, and then ask people if they'd be willing to practice.
I don't know how you've asked, but you say that you can understand most conversations. It might be worth it to point this out: that you can understand them better than they may be able to understand you. This may alleviate some of the weariness of them being misunderstood by you, of having to explain themselves multiple times in what is supposed to be 'just a conversation'.
Also, a bit off-topic but maybe a taalmaatje might be able to help you already. These are people that volunteer their time to teach people Dutch, through having 'just a conversation'. If your school/university has a notice board or something, asking for a volunteer there might help too, and if you haven't yet, it might be worth looking into whether the school offers classes or other resources for foreign students as well.