Cirno,
First, you are working with a toy that I would suggest is much too wide for a beginner. You can get things that are 1" or even 1/2". Think like http://www.squarepegtoys.com/shop/depth-probe/, the 1"x15" would be a great place to start. Another option might be something like http://www.squarepegtoys.com/shop/slink/.
Second, you need to understand your anatomy. If you don't, you are just pushing and pushing and not getting anywhere. The turn from your rectum into your sigmoid is a pretty tight turn, and doing what you can to relax and straighten out the turn is critical. I tend to lay on my left side, with my left leg straight and my right leg bent. I also arch my back and twist my hips somewhat to ease the angle. The correct angle must be achieved to even get your toy close to its target. You go in your butt, but then you really need to turn and head toward your spine for 6-8 inches following your tailbone to get to the junction with the sigmoid, and then it turns sharply to your left side. At maybe 12-16 inches, it will turn up your left side into your descending colon. I've never been able to get past my splenic flexure into my transverse colon, so you are on your own past there.
Third, you have to get cleaned out or you are going to be trying to pack a wall of feces back up your colon, which isn't healthy or comfortable. You said you used an enema, but generally, the little bulb enemas aren't going to get very far up into your colon. I generally find that I need to put 3 or 4 half gallon enemas up and expel them before i feel like my colon is empty and ready to accept something foreign. Then I prefer to sit for an hour or two, have a stiff drink, let my colon soak up any remaining water, urinate, and then I play.
Fourth, lube, lube, lube, lube, lube. To give you an understanding, I buy lube in gallon jugs. Standard ID glide is my preference. If you shop around, you can generally get a gallon for less than $50 on the net. Generally I expect to need roughly 1 gallon per year. Think of it as a gradual process, first use your toys to get lots of lube into your rectum, its okay if there is too much, the more the merrier, and the less likely you will hurt yourself. The farther up you go, the more lube you need, keep removing the toy, putting more lube in your ass and on the toy, using the toy as a piston to push more lube up there.
Fifth, be careful please. Noone wants to be the sucker in the emergency room trying to lie about how they got an 18" double dong lost up their bum. For the record, if that happens to you, don't even bother lying, the ER people darn well know what happened and they don't have time for you to be embarrassed. If you feel pain, slow down or stop. If you see a bit of blood or red mucous, stop for a few days. If you see more than a little blood, go to the hospital and be frank with them, you are protected by privilege, so they cant tell anyone. Don't try to force anything, if you puncture your rectum or colon, your life will suck for quite a while.
Sixth, and you got this one right already, ask for insight and/or help if you are unsure but still want to continue. Many of the people on this board are extremely experienced and can offer huge insight. One thing I would recommend is watching videos of other people that can do this, namely porn stars like: HotKinkyJo, Amber Rayne, Roxy Raye, or the legion of gay men that post similar videos. If you are looking for experienced help, I'm sure whether you prefer boys or girls, you'll find volunteers are legion on this forum.