If you don't have good control at this time, you might want to stick close to the toilet, but not in a squatting position. I find lying on my left side to be comfortable, then turning onto my back after half the water is in. On my back is a wonderful position for me. It's very relaxing and I can hold for quite some time in that position.
What do you think is causing your need to expel?
Does it feel like the water is blocked and has no choice but to rush back out? Attempt a normal bowel movement to cleanse the lower portion of the bowels. If you are constipated, before trying an enema, try using a glycerin suppository to get things moving inside. If that is ineffective or if it is effective, but you want a larger enema, try using a bulb syringe. It is easier to control the amount of water and the speed of entry. Taking smaller enemas to start will make holding a larger enema longer easier to accomplish.
Is it cramps? If so, what solution are you using? Are you starting with something gentle? What lubrication are you using? Certain solutions and lubes can stimulate the urge to push. Using a simple water and salt or water and baking soda enema and a water soluble lube like K-Y jelly.
Water that is too warm or too cool can cause cramping, so try different temperatures to suit your personal enjoyment. I can more comfortably hold a cooler enema, but many others here have better luck with a warmer enema. So experiment and find your body's preferred temperature and comfort level.
Is there air in the hose? Before inserting the nozzle, always run a little water through it first to push the air out, just as nurses do to syringes before administering injections. Air in the hose can cause pressure to build up in your intestines and will make holding water difficult as the air wants to come out.
Does the water flow too quickly inside you? If so, your bag may be too high or your hose may be too wide. Lower the bag and/or use a hose that is narrower than your current hose. Are you taking advantage of the clamp? Try taking in a small amount of water, clamping the hose and getting comfortable with that amount before unclamping and taking in more water. The clamp can be your best friend.
Let me know how things work out. If none of these ideas help, we'll find someway to fix it. Good luck and keep us posted.
Dianne