as I've met you does not sound natural to me. The person that you are speaking to knowe when you met, so now is redundant. It would be better to combine the ending of the sentence to make until I met you.
I've been feeling lonely... until I met you. - present perfect
This suggests that you have been feeling lonely for a while, until you met the person that you are speaking to. This meeting just happened.
I'd been feeling lonely... until I met you. - past perfect
This suggests that you had been feeling lonely for a while, until you met the person that you are speaking to. This meeting took place at some time in the past: the person you met will know when it was.
I was feeling lonely... until I met you - past simple
This suggests that you were feeling lonely when you met the person that you are speaking to. It does not indicate whether this feeling of loneliness had been going on for a while, or whether it was just a temporary thing just before the meeting took place. This meeting could have just taken place or could have happened at some time in the past: the person you met will know when it was.
If you want to say until now, the present perfect or past simple would work. You can't use past perfect, because that would only work if the meeting took place in the past.