I understand that we cannot take this relationship any further.
I know it is difficult for you to be in a relationship with me as you already dating somebody.
付き合うの意味は:'to date somebody', 'to go around with someone'
いま誰かと付き合っていますかの英語は以下の通りです:
Are you seeing someone?
Are you going around with someone?
Are you dating somebody?