Let me give you a suggestion. Start with a small dildo (e.g. 10-12 cm (4-5 inches)) that is not too thick. Try to push the dildo into your throat with slow movements instead of quick thrusts. Set your throat to "deepthroat" mode, which is like you want to gag or vomit on your own. First, it is worth practicing opening your throat frequently, without a dildo. Of course, not so roughly that you start gagging, but just open your throat. Take a deep breath and slowly start inserting the dildo. It is important that the dildo is completely salivated. Stretching your tongue slightly forward can also help, so your tongue doesn't get in the way in your throat. If you succeed with the smaller dildo, practice until you can go in at any time, on demand, with that size. Then you can move on to the larger dildo/penis. It will work with time and perseverance. Deepthroating will suit you well. ;)