I don't think you can force yourself to be little. If you don't want to do it, it's just going to feel awkward and like you're acting. I also feel there are different ways to be little and to varying degrees. For instance, I have a onesie, a paci, a bottle, and some stuffies. Using them all gets me into little headspace, but even then, I don't feel too regressed. So don't panic if you don't feel little enough. Also, you don't have to go with every aspect of being little. For instance, your big mentioned tantrums. There's no way I could do that. I can't stand it when actual kids have tantrums and I couldn't do it in little space, nor could I tolerate a tantrum from an ABDL deep in little space. Just do what feels natural to you.