One thing nobody mentioned: you're probably studying in a way that doesn't match how you actually learn. Like, apps and videos are great, but if you're the type who learns better by talking things through or writing stuff down, it gets boring real fast. Try figuring out your style - there are people who tank their motivation even more by forcing a method that just doesn't work for them.
Now here's a practical hack: instead of thinking about "maintaining discipline," try turning English into something you're already doing naturally. Change your phone's language to English, put English subtitles on the shows you watch (no dubbed audio), follow accounts that post about stuff you're into in English. It's like you're "studying" without actually sitting there with a textbook or app. It works because it doesn't feel like an obligation, it just happens throughout your day. Start with things that are actually cool to you, not heavy grammar.
And I know people have talked about routines before, but the real deal is that a routine without flexibility kills your motivation too. If you planned to study for 40 minutes every day but woke up too exhausted, just start with 15 minutes. That's not failure, that's being realistic. The people who keep this up for years aren't overly disciplined, they're disciplined enough to get back on track when they slip, without the guilt.