I learned a foreign language relatively late in life. I was 30 years old and I've had the same problems as many of you - starting and stopping, motivation coming in bursts and then dying off again, getting a beginner's boost and then hitting a plateau. Language learning is hard! But it's not impossible. Below are a few things that helped me the most. There is one overriding learning method I used that really helped me, much more than anything else I have learned, and I cannot recommend it highly enough.
I wrote about my learning experience and how I started Deutsch Gym at length, so go check that out if you want to learn about the details of starting a community for the first time.
Intensive courses were a game changer for me. I had previously done 1 month of A1 after-work classes, 5 years before. I did a month here and there of private lessons, and add in some iTalki lessons and keeping that Duolingo owl happy and you have the typical impatient beginner package. So essentially 5 years after taking my first German classes I didn't have a whole lot to show for it.
Then at the end of 2019 I was living in Berlin with no job, and decided it was time to really learn German. I pushed off getting a job and decided to commit to learning German 100%. At the last minute, I rang a German school on the first Monday of November and enrolled in a B1.2 month-long intensive course. I showed up to class on the Tuesday, the 2nd day of the course.
I did the B1.2 intensive course for November, then I did B1.1 in December (my overconfidence in my abilities led me to enrolling straight into B1.2 - I don't recommend this!). I took a break for January and then did B2.1 in February and B2.2. in March.
For the first 2 months I didn't have a job, so I treated learning German as my job. I would have a class from 9am-12pm, then I'd head to the Berlin State Library (amazing facility), get lunch and coffee in the cafeteria, and study and do my homework for another 3 hours. For the last 2 months I worked 20 hours per week whilst attending the course. It was busy but manageable.
Goethe is expensive, like really expensive. I just checked out their website for the cost of an intensive course. The course consists of 80 lessons x 45 minutes = 60 hours of learning. The cost is €1099. That works out of €18.30 per hour.
My school, Kapital Zwei in Berlin, cost €225 for a 1 month intensive course, and you need to buy a book that costs €30. So the total is €255. They are 4 days x 3 hours of classes per day. That is 48 hours of learning. This works out at €5.30 per hour, less than 1/3 the cost of Geothe. I have no experience with Goethe - I'm sure Goethe is good - they have a great reputation, but you can get 3 months of classes in a normal school for the price of 1 Geothe month.
I was happy with the quality of teaching in my school, so that's what I recommend.
There were between 7-10 people in my classes - you definitely do not want more than this. Packing people in is great for the profit margins of the school, but not so much for learning.
You will not get a lot of speaking time in the class, nor will the teacher have a lot of time to spend on you individually.
If you don't do the homework and don't make an extra effort to speak a lot in class (you actually have to be a bit pushy as there are so many people in the class and time is so scarce), you will sit in silence for a month and not learn a thing. One of my classmates who did exactly this was on this 6th month of intensive courses, had a German-speaking girlfriend, and was still not able to speak properly. You have to meet the universe halfway if you want to learn.
In my first month the teacher used the book heavily - it was really like a guided course of the book, interspersed with her drilling down on certain grammar. I have to say this strategy worked pretty well. It really made me realise that the exercise books are actually very good, and how important it is to do the homework they give you.
The homework was really important - the books they give you have had decades of fine-tuning the exercises to test you and make you practice exactly what you should learn. They cover every bit of grammar that is in the course. The more homework you do the more you will learn. I can't overestimate the importance of doing the homework enough.
As I said before, you will not get a lot of speaking practice in class. I was lucky here as I could speak German to my girlfriend in the evenings. At the start we started off with just 10 minutes in German - anything more would tire us both out. Then we eventually increased it to 1 hour at a time, and then to an entire evening of Friday drinks only in German.
If you're not going out with a native speaker it'll be harder for you, you'll have to seek out partners from your class to practice with and look online for meetups. I started Deutsch Gym in my 3rd month of the intensive courses as I need practice outside of speaking to my girlfriend, so that is how I improved my fluency a lot. However you find someone to practice with, you will have to - I consider this a must.
Those are my tips on intensive courses. It's a struggle at times but it is really worth it. Best of luck!