Hi machiruda!
I'm so sorry to hear you've been struggling with this - it sounds frustrating. Pleasure is complicated and sometimes unpredictable so there's never really one "right" answer of how to find what works for you but there's a lot of things that can impact it that we can try out.
In general, with sex of any kind, your mood can greatly impact your ability to experience pleasure. If we are stressed out, maybe from the environment we are in or maybe from wanting it to feel a certain way, we are less likely to relax and enjoy what we are feeling. It does not mean you are broken or things are wrong but it is really helpful to note. When you are trying to masturbate, it sounds like you are a little stressed about wanting it to feel a certain way and that could very well be gettting in the way of you being able to relax. I would recommend setting a calm enviorment and exploring pleasure with no end goal in mind. It might help you find a more calm approach to pleasuring yourself. Some things that can help with that are engaging your other senses, smells you like, music that calms you or feels sensual, even tastes can help you set up that mood a little better. When you are reaching that point that frustrates you, what are you finding yourself thinking about? Trying to identify what you are feeling can be helpful in reducing stress.
That's a mental component of masturbation. There are a lot of physical components we can try out too! You mentioned you don't seem to find any pleasure from "touching my breasts, thighs, stomach... Anything except my clit and the inside of my vagina (if that makes sense), even when I'm really aroused." That's completely normal! All bodies are different and experience pleasure differently so don't worry about not feeling it in certain parts of your body. For example lots of people don't have sensitivity in their breasts. It's great to hear you have been able to identify what DOES feel good for you so I think that's a good place to start. There are a lot of different ways to masturbate, which you can read about in these two great articles:
Going Solo: The Basics of Masturbation How Do You Masturbate . There's a possibility you might be feeling oversensitivity and trying to masturbate through a piece of clothing could be helpful. You mentioned you don't own any toys, maybe looking into trying a vibrator might be helpful in pushing you past those seconds you mentioned. Maybe try moving at a slower pace or a varied one allowing for more of a build up. I would encourage you to read through these pieces because they might have some helpful suggestions for what you're struggling with.
Overall, I would try to remember that masturbation doesn't have to feel a certain way and it doesn't have to look a certain way; there are plenty of people who don't enjoy masturbating at all. Some of them love sex with another person and some don't like it at all. I understand your frustration of wanting the pleasure to culminate in something AND remember that a lot people don't experience orgasms or things like that. If you can maybe try to focus on the part that you do enjoy instead of what might come next, it might expand that period of time where you feel pleasure.
Let me know what you think! I hope this helps. I'm sorry its no easy fix but there's lot of things for you to explore.
Best,
Val