你的句型是错的,应该是：
I was having a good/happy/great time with you.
= I was enjoying myself with you .
= I was having fun with you
根据汉语意思,用一般过去时更好,即
I had a good time / happy / great time with you .
= I enjoyed myself with you .
=I had fun with you .请问为什么是was +doing 的形式呢过去进行时呀，它的结构是was +DOING感觉没有必要用过去进行时吧,如果是过去式呢,怎么说所以我前面说到用一般过去时更好呀。即

I had a good time / happy / great time with you .
= I enjoyed myself with you .
=I had fun with you .