Listen closely. Never, at any point, should a woman be telling you what you can or cannot do with your own body (outside of a few things in marriage, like sleeping with other women, for instance). Now, she is permitted to disagree with whatever she wants, but if this is something you really want to do, you need to lay down the fucking law and let her know whose boss...and I don't say that in a domineering kind of way, as I don't think men should "boss around" women, but I do mean that you need to reassert your leadership in this relationship...because at this point, you have proven that you are submissive and afraid or her. Otherwise, you never would have hid your stuff in the first place.
What you should have done is sit down with her BEFORE you started using...and told her clearly and coolly what you were going to do and why you were going to do it....and then just do it. Of course, she would've probably tossed around some threats, possibly even threatened to "leave" you (note: to this day I don't know a single woman who has ever left a man she loved because he decided to use steroids, but I've known plenty who have gotten really pissed because the person they thought they could trust hid shit from them). She may not like your steroid use, but she will accept it as part of who you are, but ONLY if you become the leader. If you allow the woman to lead--to call the shots--not only will she never be happy with you (because women don't actually want a pushover who cowers before them), but you will never be happy. However, in exchange for leadership you must act in her best interest. This doesn't mean giving her everything she wants, obviously, but it does mean putting her needs ("needs", not necessarily "wants") before your own. It's hard to do this, but well worth it in the end. If you had already been doing that, this steroid shit you're dealing with now never would have happened. Just so you understand, I understand where you're coming from...because I was there myself 15 or so years ago, but since then I've found a better way to live--a far superior way of relating to and dealing with women. Everyone ends up being more happy.