I don't think lying to your therapist is helpful. I can understand your fear that you may have to go back to the hospital and miss your friend's wedding - that would be horrid! But, perhaps it may be more helpful to gain support and techniques and skills from your therapist rather than lying? You have something you want to stay out of hospital for, so that's a good incentive. It is understandable you don't yet have the skills to prevent yourself from self harming in serious ways - it is a lot to just stop when you're mind is set on it. I wouldn't feel guilty about not knowing how to prevent it. But, as hard as it is, I'm always of the opinion that if I want to accomplish the things I want to - like you going to a wedding - it is important to bring in extra support during those times so I can reach that goal. In addition, I find everyone tends to worry a heck of a lot less when I'm honest and open, then they can truly risk assess appropriately and this prevents what I may consider 'over-reactions' due to limited information. Doing this may allow you to feel as though you have taken some control back over your treatment plan rather than feeling the only way to do so is to hide it and consequently feel guilty about it.

Ultimately it's up to you how you utilise the support of your therapist. I'm sorry you are in so much pain that this is the only option you feel you have currently. It is hard to communicate how hard it is, and manage the fear that comes from doing so.