Hey, so I saw your post, and I thought, yeah, I know how that feels... The thing you really have to do, when trying to learn the tweener, is patience... It took me almost 7 months of just doing it over and over again before I could hit it in consistently, and another 3 until I could do it in actual matches, and I'm #1 singles on JV for my team...
It is ALL REPETITION, and as you do it more, you can definitely get more comfortable... Just tossing it 6 or 7 feet away from you, running to it, and hitting it between your legs is great practice for the first couple months... These are 2 videos that I did on specifically the tweener:
Check those out, and I'm doing one this Tuesday as well, so subscribe, so that you'll be able to see it!!!
Hope that helped!