Agree with you on exercise. But staring at a painting is to meditation what curls are to building your core. People all recommend the same type of meditation for a reason--so that you can close your eyes, focus on something, notice when your mind drifts, and bring the attention back. You didn't mention anything about awareness or noticing when you lose focus, so your advice seems to completely miss the point.

I did say that starting with something active may not be the best introduction if you want to improve your general ability to control your attention. That said, it does work for many people including myself since I became aware of the ability to train my attention through both programming as well as weightlifting. Funnily enough doing bicep concentration curls is something that has all of the aspects of a meditation practice for me and where I can really find a complete and singular focus and exclude all other thought and distractions. Building mind-muscle connection naturally leads you to focus solely on the muscle you are working, the sensations of adduction and abduction, the proprioceptive sense of maintaining the same path for the weight to travel and keeping the muscle under tension and flexing it fully. If you're doing it right then you are focusing on nothing else, you're not thinking of anything else, you have no inner monologue other than counting reps and sets and it becomes a flow state.

You can only tell people so much in words about how to maintain and train their attention. The better way is to learn by doing and it's easier for people to learn when it involves doing something agreeable to them rather than forcing them to do it the way you did. The original poster said they had tried the standard way many times even with help from a therapist and they didn't make any progress. That's why I felt it necessary to point out that there isn't just one dogmatic way to do it since your attention can and is applied to many different things and just like using your muscles and body to do many different physically demanding things you can train it in a variety of ways.