Just saw this question while scrolling through, and really felt the need to answer. Don’t worry, 25 is definitely not too old to get into the industry, it’s nowhere near too old to get into the industry. People still enter the games industry in their thirties and still have successful careers. I work with people who are in their forties and fifties (actually, I’m not that far off forty myself).
I know it can be difficult to get that first games industry role, I’ve been in that situation myself. I graduated university in a city where there was a very high quality game programming degree and as someone with a normal computing degree I found it very hard to compete with the graduates from that other course. I ended up taking a job in testing, and then a couple of years in design while I hit the books to fill the gaps in my knowledge, and develop a strong portfolio piece to land me my first game programming job.
Actually funnily enough, while I did have a little bit of games industry experience at that point, I didn’t get my first entry-level C++ programming role in the industry until I was 25. That was 14 years ago, and I’ve been programming in the AAA sector of the games industry since then, worked on many well known titles (some great, some not so great 
). Don’t give up!
All I can say is that I know it’s difficult, and that sometimes it can be very hard not to get discouraged when you get a knockback from a games company (or sometimes not even get a response), but try to view every setback as a learning experience, any gap in your knowledge as an opportunity to improve (and I know this is easier said than done). Even once you get into the industry, you’ll still have the odd setback here and there it’s just part of life, a job interview that didn’t go as well as you’d hoped, a cancelled project, a layoff, it happens to the best of us.
If you’re having problems learning/retaining game-specific skills, try and figure out what works for you, whether that’s video tutorials, courses, books, etc. Personally I found that sometimes a book was worth a thousand online tutorials, but there are far more options out there nowadays. When you are trying to learn, try to avoid that feeling of discouragement/panic, it’s not conducive to learning.
Ultimately, just keep at it, remember that a solid portfolio piece that demonstrates that you can make a quality polished product yourself, and that demonstrates ability in C++, game-specific maths skills, can break down so many barriers.