First off, you probably want to figure out if your nihilism is due to philosophy or depression. Would you normally enjoy and value things, but idea of finite life gets in the way? Or would you have difficulty seeing a point to things even if you were suddenly granted immortality and the heat death of the universe was averted?

Either way, it's difficult to give a definitive solution, as different things work for different people. That said, if the problem seems to be philosophy, it might be worth noting that the satisfaction found in a good moment isn't a function of anything that comes after it. If you enjoy something, or you help someone you love, or you do anything else that seems valuable to you, the fact of that moment is unchangeable. If the stars die in heaven, that cannot change the fact that you enjoyed something. Another possible solution would be trying to simply not think about it. I know that sounds horribly dismissive, but it's not meant to. In my own life there have been philosophical (and in my case religious) issues that I never managed to think my way out of... but when I stopped focusing on the problem it went away. I managed this only after getting a job that let my brain say "okay, worry about God later, we need to get this task done first!" If you think it would help, finding an activity that demands attention might help (if you feel that your brain will let you shift your attention; if not this might just be overly stressful).

If the problem seems to be depression, adrafinil and/or modafinil are extremely helpful for some people. Conventional treatments exist too of course (therapy and/or anti-depressants); I don't know anyone who has benefited from therapy (at least not that they've told me), but one of my friends had night and day improvement with an anti-depressant (sadly I don't remember which one; if you like I can check with her). Another aspect of overcoming depression is having friends in the moment and a plan for the future, not a plan you feel you should follow, but one you actively want to. I don't know your circumstances, but insofar as you can prioritize socialization and work for the future, that might help.

As for the actual question of self-improvement, people vary wildly. An old friend of mine found huge improvements in her life due to scheduling; I do markedly better without it. The best advice I can offer (and this very well might not help; drop it if it seems useless or harmful) is three things:

Don't do what you think you should do, do what you actually want to (if there isn't anything that you want, maybe don't force trying to find something too quickly either). People find motivation in pursuing goals they actually find worthwhile, but following a goal that sounds good but doesn't actually excite you is a recipe for burnout.

Make actionable plans-if there's something you want to do, try to break it down into steps that are small enough, familiar enough or straightforwards enough that you can execute the plan without feeling out of your depth. Personally, at least, I find there's a striking "oh, that's how I do that" feeling when a plan is made sufficiently explicit, a sense that I'm no longer blundering around in a fog.

Finally, and perhaps most importantly, don't eliminate yourself. That is, don't abandon a goal because it looks difficult; make someone else eliminate you. This is essential because many tasks look impossible from the outside, especially if you are depressed. It's almost the mirror image of the planning fallacy-when people commit to doing something, it's all too easy to envision everything going right and not account for setbacks. But before you actually take the plunge, so to speak, it's easy to just assume you can't do anything, which is simply not true.

First, this sounds tough, hope you're doing okay. I hope eventually you find meaning in a wider variety of things that give you a more robust sense of purpose, enjoyment and motivation.

My 5 minute answer is:

First, consider that you may be depressed. If nothing seems meaningful, it may just because there is a chemical imbalance in your brain, and that fixing that is way more important than any particular skill, goal, or psychological healing.

Second, I'd start by taking of anything that has even a hint of "feels good/yummy/enjoyable/worthwhile." There's a lot of skills you can build to keep focused on things that develop your longterm character, but it's way way easier the closer your actions relate to things you actually enjoy. It sounded like you don't find joy/value in most things, but is there anything that.... I dunno, even remotely feels satisfying to do?

Most or all of these ideas are things I have or am trying or are variations on them:

You could send a recurring email to yourself everyday (using Mail Conductor if you use Gmail). As part of the email, ask: how can this recurring email be improved? Have I done [x] that is relevant to domain of my life [y]? For example: Did I do exercise today? Have I read a book? Have I written? etc. And include hopeful beliefs that you want to remind yourself of, or of exciting goals you want to accomplish.

Likewise, you could make a personal homepage with these reminders.

You could use the app Tasker if you use an Android phone to pull up one of these things as well, for example whenever you open your phone.

You can track your habits using Beeminder, using the Lights Spreadsheet, or the visual dashboard.

You could post this question to other places on the internet. (there are many such places!)

It's totally possible to build a support network of people online. You could join rationalist Discord servers and build a support network there, perhaps making your own Discord server (pretty easy). There are so many people out there who are in fact willing to talk.

You can carry around a notebook and just write your thoughts in it, or journal your thoughts in a simple text file.

You could write out a bunch of your negative thoughts in a notebook, then write out more functional/truer thoughts, put those on index cards, and review those everyday. Or just put hopeful thoughts/reminders on those cards, or quotes from people you look up to.

Scheduling: You could try setting a 15 minute timer to figure out whatever's blocking you from using a calendar. You could figure out how to use voice commands such that you could be like "Yo calendar, at 5 pm I will spend an hour writing a LessWrong post" like it's a human.

You could try taking the meaningful qualia you feel around the girl you're infatuated with and learning how to autogenerate and attach it to other things. (If you spent 10 minutes on it, could you fall in love with a cup?)

You could spend 15 minutes on the regret-minimization exercise

How long do you go outside? Do you get sunlight? Do you ever go in the woods?

Do you go to any events or meetups? You could try the intention of going to one three times a week.

What activities did you enjoy as a child? You could try doing that several hours every day, guilt free.

Every night, you could write out a story of how you want tomorrow to go.

You could do daily email reports with an accountability partner. I would be willing to be your accountability partner if you wish.

Let me know how things go!