Hi Jeroen,
As you can see, you're not alone in feeling frustration from time to time, as everyone of all skill levels can get stuck. You've got a great attitude, passion and patience to persist, so I don't think you'll be stuck for too long.
There's no need for me to reiterate on the excellent advice you've already been given. For me, when I get stuck, whether it's with learning guitar or just learning for my career, there are a few things I do to help myself.
1) Obviously persistance, which you have - so don't give up. Learning is a lifetime experience, you're in this as a marathon, not a sprint.
2) Get different perspectives - You've started doing this already, by voicing your frustration and asking for advice. Guitartricks isn't the only source to learn from, so along with your practice and things here, branch out to some freely available resources online. YouTube for example has endless videos if you know what you're searching for - fall down a rabbit hole and see how others do things. You'll hear and see a different perspective on the same thing. This motivates me all the time.
3) Do something different - others have mentioned this as well, but try something different. Move to the next lesson, play a song, fiddle around and goof off etc. Siloing yourself to just the same exercises add to your frustration. "Work smarter, not harder" is the idea. Obviously you're already working hard, you just need to jolt yourself out of the "trying too hard" loop. I think this will help you relax and smile more, even when you make a mistake.
4) Practice should be fun, not a chore - we all have off days, some may argue we have more off days than "on" days. It happens! If you're feeling like it's a chore, everything becomes more difficult. Recognizing when something fun becomes a chore is part of life experience, and I think when you're aware that something isn't working, that's when you can change things up.
You're doing fine! The good news is that you're never alone and this community will support you. Don't give up, and have fun!
-Eric