The anal muscle is naturally closed. YOU have to learn to relax it and keep it relaxed which isn't something your partner can do for you. Though your partners may have not taken the time to help you relax, which some like to do but others don't. So it is up to you to learn how to do it. Get a small dildo and a very good lube or even shortening, like Crisco. Get the arse and the dildo very slippery and try inserting it very slowly. Take your time and relax, relax, relax. It feels like taking a crap the first few times but that feeling goes away. Just keep remembering, relax. It is a learned thing, not something your body does naturally. Get some porn you can watch and let yourself get distracted. Once you've figured it out with the small dildo, you can get a larger one but don't immediately go for giant. Remember, your goal is to be able to enjoy a real guy. Bigger than that isn't helpful. One last thing, the more often you do it (the dildo or the real thing), the easier it will be. Between times, the anus will contract back and the harder it will be to relax. But practice will help you learn how to relax. Have fun!