I was very much the same way. For 20 years all I did was body part splits hitting each muscle once per week. For the past 5 years I've been incorporating a variety of training splits into my yearly plan... and it's the best thing I've ever done. I find that each split with various frequencies can really feed off each other.
Right now I'm doing a 4 day body part split routine (hitting each muscle group once every 5 days) that looks like this;
Monday: Chest & Triceps
Tuesday: Legs
Wednesday: Shoulders (plus touch up work for my chest. light pump work)
Thursday: Back & Biceps
Friday: Off
Saturday: Repeat split
My body is responding really well to this, especially after coming off a high frequency training plan where I was hitting every muscle 5 days per week.
Monday: Full Body high rep sets, typically utilizing a protocol such as Myo-reps. 60 total reps per body part.
Tuesday: Full Body 4 sets of 3-5 reps for each body part.
Wednesday: Full Body 6 sets of 10-12 reps for each body part.
Thursday: Off
Friday: Full Body 6 sets of 6-8 reps for each body part.
Saturday: Full Body 6 sets of 8-10 reps for each body part.
Sunday: Off
I really enjoy this split...
Monday: Upper - strength focused
Tuesday: Lower - strength focused
Wedneday: Off
Thursday: Push - hypertrophy
Friday: Pull - hypertrophy
Saturday: Legs -hypertrophy
I also enjoy...
Monday: Push - strenght focus
Tuesday: Pull - strength focus
Wednesday: Legs - strength focus
Thursday: Off
Friday: Push - hypertrophy
Saturday: Pull - hypertrophy
Sunday: Legs - hypertrophy
I also perform...
Monday: Upper - light weight high reps
Tuesday: Lower -light weight high reps
Wednesday: Upper - heavy weight low reps
Thursday: Lower - heavy weight low reps
Friday: Upper - moderate weight
Saturday: Lower - moderate weight
All splits have their pros and cons. By incorporating a variety of them into your yearly plan you maximize the benefits from each of them.