It might be worth trying a slimmer and longer, more flexible toy as an alternative first. Say an 18” double headed dildo. It’s not about getting 18” in, but easier handling. The idea being to let it ‘find its way’. It might turn back, it might nose its way in. Toys like this can be a real handful if you’re using lots of lube-which you should. Just use a facecloth and then you’ll have a good grip and great lubrication at the same time, and have more length to hold on to as you explore.
Another approach as an alternative to filling some air in is to use water. After you’re confident you’re good and clean inside, run some water back up inside until you feel comfortably full. Then ease the lubricant-covered dildo up against your butt and gently push it inside. The dildo/snake/cucumber will be surrounded by water in your gut. This can be enough to hold the opening of the sigmoid a little wider and so allow the tip and more to enter.
This is a balancing act of clenching and relaxing, only safe standing in a shower or bath as the displaced water will eventually want to escape. Also stand on a play-towell and or brace your feet and watch your footing as the mix of lube, water and bathroom ceramics can be shockingly slippery.
Once the toy is in, you can relax and let more and more of the water out and you’ll start to feel the gut walls close in and meet the shape of the toy. For the water approach you should use a non water based and toy-friendly lube to maintain slip inside.
Once you’re able to play without starting with a fill of water, then move back to water-based lubes.
Be super gentle whilst your body gets used to this. Then practice. And enjoy!!!