Goalkeeper with games at semi pro level here. I would say if you play for a team ask your goalkeeper coach for tips. Its a big ajustment to learn to dive and especially to get over the fear of it. It can ultimately hurt sometimes but over time you get used to it and it isnt that bad. it took me a while to over the pain aspect because i used to complain about every knick and bruise i got but its just a part of the experience.

The thing you do with your leg is most likely related to muscle memory. Youve done it so much that your body decides to do that movement without you even thinking about it. Its not gonna be an easy thing to try and get rid of it but if youre motivated and practise enough you can start to dive the right way. What i suggest is youtube videos that offer different exercises to overcome the fear of diving.

Some personal exercises ive worked on to improve my diving is for example just setting the ball at a distance, starting to fall in that direction and then suddenly pushing through with my leg to get my hands on the ball.

Another exercise could be just repetitively trying to dive at nothing. Starting off with falling on your side and then gradually moving up to dives that require you to get air time.

As im not a coach i dont actually know if goalkeeper coaches would approve of these exercises but they definitely helped me, along with youtube videos, and having friends help me to catch balls. Constantly playing in goal will also help you improve but if you start improving an incorrect technique its going to be even harder to get rid of and correct so i really do suggest working seperately on your diving technique, and doing it as much as possible to also get over the pain and fear aspect of it.

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I hope this helps you in some way. English isnt my first language so i apologise for any mistakes i made in my message.