See a sexual therapist together or alone?

Therapy is something that is best sought together as this can enable you both to share your opinion, and for each of you to benefit from the knowledge of a professional. If you go on your own then only one of you will be profiting, and whatever you learn will likely fall on deaf ears when you try to involve your partner, though this is not always the case. If your partner does choose not to go then you should not allow this to stop you. Even if you feel that relationship problems are down to your individual faults it is still best to be accompanied by your partner, as they will be able to offer you support and assistance.