No, I wouldn't. I don't feel the need to apologize for such things, as I certainly wouldn't expect or even particularly appreciate an apology from someone else in the same circumstance - because a person's mood does not wrong me. I have texted the therapist twice to apologize for actions. In the first instance, I had broken an agreement we had made, and in the second, I had been unjustifiably rude to her.

It sounds like your therapist is good with you texting, so that doesn't seem like an issue. However, I think most therapists would take exception to the idea of apologizing to them for having a low mood. That just seems kind of silly. If your low mood made the session unproductive, the only one suffering the consequences for that is you. While I'm sure it's more rewarding to have a productive session from the therapist's perspective, I don't think I'd assume unproductive sessions are "difficult." If they do experience it that way, the therapist needs to deal with those feelings of frustration or whatever, and separate themselves emotionally from whether the client is in a place to do work on a particular day or not. Finding it difficult to work with a client who is experiencing low mood and not able to contribute as usual makes therapy about the therapist and the therapist's needs, not the client.

So I wouldn't apologize. It is not your responsibility to make sure your therapist feels fulfilled by sessions with you. If you want to talk about your desire to apologize, that could be interesting - or how your mood got in the way of a productive session. I just don't think you have anything to apologize for.