Let's stop and think for a moment here. You want to numb up your ass so you can do anal. So, how are you suppose to experience the pleasure of anal sex if you can't feel anything? Sounds like you are defeating the purpose of anal sex from the very beginning to me.
Also, it sounds as if you are having some bowel problems too. When it hurts to have a bowel movement, something is wrong. Do you drink a lot of milk and eat a lot of cheese? Dairy will constipate you which is what it sounds like to me. You may want to change your diet and include eating lots of fiber to make getting clean on the inside much easier. I am not a physician and recommend you see one.
I always douche before hooking up with a guy. It is really quick and easy to use the Fleet enema. They don't cost very much either and can be found at most pharmacies and retail stores like Walmart.
You will want to practice accepting a cock into your hole once you know you are clean down there. You start by playing with yourself and using your fingers. Be sure to massage your prostate which is located about two inches or so up towards your navel. It will feel like a walnut.
The next step is to purchase a regular-sized
Dildo and a medium-sized buttplug. You need to know what it is like to take 6-7 inches up your butt before you are actually with a guy. Playing with a
Dildo is a good way to practice. This will also help you to determine if you need to douche before anal sex or if you can get clean by including lots of fiber in your diet.
The buttplug is used to train your ass to accept a cock. Your ass is used to only pushing stuff out of itself. Now, you want to push something up into it. You will have that "need-to-poop" feeling when you first start shoving stuff up your ass. To overcome this feeling and to learn to relax, use the buttplug. Wear the buttplug a little each day until you can wear in continuously for about two hours comfortably. I used a buttplug to train my ass and my first anal sex was WONDERFUL!
With practice and experience, you will be able to take thicker and thicker cocks up your ass. It is the girth of the cock that is most important when you are the bottom and not so much its length. Good luck!