Spent 3 years as a little fellow on non-park skis. Would jump on fat donkey rails and almost immediately slide off the other side. Then once I got park skis, I worked on a very wide and low flat rail to break the fear and get the feeling of sliding. That took a season. Then came going straight on like you are now, did that for a season. Then last year I just decided I needed to get on from the side. The big ticket for me with getting on from the side was to focus on keeping both skis parallel and under me, thinking of them as one unit. Another important part of getting on from the side for me was figuring out the optimal angle to leave the kicker, and to just expect to skip like the first 1/4 of the rail during your mounting jump. Also, I found it much easier to learn coming on from the side on round rails, I find it easier to lean either way to adjust if you don't land perfectly centered.
Now I've got that down, spent last season learning fun-shaped rails like down-flat-downs, rainbow rails, zig zag rails, etc. This year I'm hoping to learn lip on and getting on from switch.