しばらく日本に戻らないので付き合いのは難しいです。
"付き合う" rather than 付き合い noun vs verb
もし日本に戻ってもそういう関係は良くないと思います。
"良くない" does mean "not good" but, in this context, could come off badly. I'd probably stick to "もし日本に戻ったとしても付き合うことはできません。" It's direct and a bit harsh but it doesn't make a judgement of the relationship. (Also "戻ったとしても" means "Even if I return" which does make it more definitive.)
That said, if it were me, I would keep it more simple than what you've written.
"気持ちは嬉しいですがあなたとは付き合えません。