Two more points. Ever tried stretching? It does not matter what king of stretching: for splits, for deep-throating, for dildos or butt-plugs. There are two cornerstones: relaxation and breathing. If you stop breathing, you will tense, if you are tensed, the contracted muscles will not allow you to go further.
And if you do not know how to relax, there is a good method – you have to tense the corresponding muscles with all your force for, say, 20 seconds, then relax, then repeat. The muscles will eventually tire and relax. And this is the moment to apply some stretching force (but not too much!)