When I play, and plan to go deep, I always do an enema. Actually I do 4 times a 2 qt bag, so at the last bag I am all clear.
To go past the 90 degree turn of the sigmoid you have to be relaxed. Also the angle of penetration and your body makes a difference.
The softer the material of the dildo is, the easier it goes in, Its more bendable.
When you get to the "wall" feeling like you reached the end of the rectum, you will have to turn into the sigmoid. Try rotating the dildo a little and pushing in and out movements for like 1 inch, until it goes in.
Easier said than done. It took me a long time to learn to insert trough the sigmoid, and learn the feelings/sensations you experience alone the way.
Now my best, currently the only way , to insert past the sigmoid, is to sit on my knees and work the toy in, twist and turn, until it goes. Pay attention to the feelings, if you hurt stop. Don't force, try later.
I can not always get all my toys in, I have probably 8 long toys, anal snakes, double headers and the slink. The 1 inch thick snake and 12 inch (?) double header are pretty much always guaranteed to fit. Hey, sometimes it works sometimes it doesn't, being relaxed makes a difference. I can not insert them on demand, so to speak.
My advise is to try a little by little over time, don't set your mind to getting it done tonight. And yes, use lots of lube.
Happy playing, Hugs, Erin