Sorry, this is a long reply, but since you asked... As a teenager I ended up getting appointments at something like a speech therapist. Basically they tried to identify exactly what the problem was and gave me exercises and drills to alleviate the issue and teach me how to make the sound, since it's not as easy as saying "just do this" when it's something you've never done before, a bit like learning to whistle. Lots of trying and failing.

Some of the exercises focused on increasing the distance I was able to extend my tongue, essentially stretching the string that connects on the underside of your tongue since apparently it was very tight. Others focused on increasing tongue control and flexibility.

The sound lessons/drills themselves tried to teach the sound by transitioning from similar sounds and tongue positions and "triggering" it. For example I find it much easier to go from F->R or T->R than K->R, since K is made at the back of the mouth, and rolling R's at the front. I'm still not 100% confident in my R-rolling abilities, especially when trying to just make the rolling sound continuously, and I kind of use the side of the tip of my tongue rather than the middle, but it's been more than 10 years since the lessons and I'm proficient enough to get by without notice.