M.P.
I agree with Beth. I'm thinking that your boyfriend has been a part of your daughter's life for several years. She needs to be able to close her relationship with him whether or not it is a close relationship.
At 10 she is aware of your feelings and if you haven't talked to her about separating she may not understand what is going on. I suggest that you start talking about leaving before you actually leave so that she has time to get used to the idea. I don't know your daughter and so I don't know how much time she would need before you and she make the actual break.
Have you been talking with your boyfriend about how you feel. What do you expect that his reaction will be if you haven't discussed this with him? If I'm reading your post correctly you are keeping the future separation secret at least from your daughter.
The best situation if it's possible for both you and your boyfriend to keep calm is for both of you to talk with her, letting her know that you are doing this for reasons unrelated to her. The way you described it to us is a good way to describe it to your daughter. It is important to acknowledge that this decision does affect her and let her say what she wants to say. Tell her what your plans are ie; practical things such as where you'll live, so that she will feel more secure.
For example are you moving out or is he? Only share with her the things that are appropriate for a child. She doesn't need to know the details of why this happening. She just needs to know that you love her and will continue to care for her. She needs to hear that you are aware that she needs to say goodbye in whatevere way she wants to do it. Be matter of fact. Have as little drama as possible. To do this with him you need your boyfriend's co-operation.
If you separate while she's gone she'll return to a different life having had no knowledge, let alone, part in the transition. That is traumatic too. Because I don't know everyone's personality I don't know how would be best to actually talk with her but I feel strongly that she needs to know before she leaves. Whether or not you do the actual separation before she leaves very much depends
on many factors of which I have no knowledge.
You might tell her now and begin the transition. You make the adult decisions of when and how. Then include her in looking for an apartment if that's what you'll be doing. Or you can wait until a couple of weeks before she leaves so she will have an idea about what her life will be like when she returns. The most important thing is for her to know that you respect her enough to let her know what is happening in her life before she leaves. To also have the opportunity to talk over her feelings with you.
I've found that secrets create more drama in the long run. In this case I suspect she will be angry that she didn't know this was going to happen. She may feel that you've done this behind her back because you don't trust her or you think she's still a "baby." It's like something is done to change her life and she had no part in it even tho she's old enough to know at the time.