Believe me, there are very few people in this world who can turn round and say they have led a completely blameless life. Everybody has a past, and while you were single, you made the most of your freedom stop you may not have behaved in a very honourable way, but the fact that you realise this yourself, shows that you have grown up.
Many people enjoy living a hedonistic lifestyle, myself included. I have done things in my past that I am not proud of, but one thing I have never done, is tried to hide the truth from my husband and the people I love.
The truth has a habit of coming out sooner or later, and the more you try to hide things, the worse the eventual result will be when somebody starts dripping poison in this girls ear. It only takes one of your former partners to turn green eyed with envy and malice, and if your girlfriend isn't already aware of your past, the shock of it may do far more harm than anything you may tell her yourself. Remember, those who have some reason to try to destroy your relationship will not stop telling the truth. They will be very prepared to embellish and lie to make you seem even worse than anything you may have done.
It's perfectly clear that you love your girlfriend, perhaps you even see her as being your entire future. If she is to be the girl that you marry and settle down with, then she really deserves to know about your past. If she is an intelligent girl, she will see the difference between you as a foolish and carefree Playboy, and the men that she now has. You have done your partying, you have done your carousing and chasing around. Everything that you have done with this girl proves that you have become a man. In being honest with her, she will at least feel that she knows the difference between the child that you were, and the menu has become, and she can trust the man that you have become, even if she feels shocked by knowing what the child was doing. If she cannot see that, then, painful as it may be, she is not right for you. But if she can accept that the past is past, then she is definitely the kind of girl who would make an ideal life partner.
Telling the truth may sometimes be a scary prospect, but it's better than trying to conceal those things which may come back to haunt you. So, I would suggest that you choose your moment, and then sit and talk to her. If she asks questions, answer them honestly, and once she knows about the skeletons in your closet, nothing that anyone says can do damage to your relationship.
If you do have ideas of asking her to be your wife, make sure you have revealed your past to her beforehand. It would be most unfair to ask her to be your wife, and then, if she has agreed, hit her with the truth of your former life.
Very best of luck to you, and I hope things turn out well, and the two of you are very happy together.