I had that problem in college, although it was due more to my lack of sleep than boring classes. You may have gotten out of the habit of extended concentration. Focusing on one thing for long periods of time is kind of a lost skill, especially if your job or home life involves a lot of multi-tasking.

Here's my advice: make your church attendance more interactive. The obvious way is to answer questions and provide comments in class, but that's not always feasible and not everyone feels like piping up all the time. A better alternative may be to take notes. The late Henry Eyring had a tactic he used during boring talks or lessons: he would rewrite the talk or lesson in his head. I frequently do the same, but I jot things down on 3x5 note cards I keep in my scriptures. I usually like my version of the talk or lesson better than the one being given. I throw away most of those cards, but I file the good ones away and occasionally use that material in talks or lessons I have to give. And it definitely helps me keep my attention better.