You may not like the alternatives, cause it's gonna include stuff like therapy (specifically CBT) - this is proven to help. Some people who aren't medicated self-medicate with a lot of caffeine (please stick to safe levels). There is some limited evidence that maintaining a good level of fitness and visiting natural spaces regularly help, I know it can be quite difficult to stick to fitness so instead of setting aside time for fitness, I'd say integrate it into your daily life (walk instead of drive/transit for example). There is no evidence for any diet change or supplement, but that doesn't mean it can't help even a little.

Some people find body doubling helpful, I don't know actionable it is for you to have someone physically next to you for tasks is. Apparently even remote body doubling by being able to voice or video call can help.

For now, the only empirical treatment we have that is proven to work for everyone is medication - I'm sorry. There's been some limited evidence that you can train your working memory but there is not evidence that it works long term. Other than have someone else remind you - and importantly YOU ACCEPTING THOSE REMINDERS - I don't know what can be suggested to help you stay on task besides something like body doubling. I was my exes reminder person and one of the things that contributed to the break up was them being very hostile about reminders, that they asked for, so maybe show some grace to people youve asked to help when they get you back on task.

I know you don't wan't to or can't go the diagnosis route but those people are going to be able to set you up with supports. Perhaps besides medication although getting you on something might be more beneficial than nothing. You will have an easier time getting supports with a diagnosis. Yes, it will probably be difficult to stay on task/schedule but ironically, any psych worth their salt would see that struggle and conclude you likely have ADHD lol. It may be easier if you are at a university that has a health clinic for the student population.

Without medication, you are likely going to have to rely on friends and family for some amount of time. You are probably going to feel embarrassed asking for this help. You are probably going to feel anxious about rejection (adhd is very linked with rejection sensitivity, which is a much smaller word than how being rejected/failing feels to someone with it implies). You are going to have to get past any embarrassment and anxiety you may feel because battling this alone is a mugs game. It is hard to ask for help, you are going to have to anyway.