How to pronounce [ɹɾɚ] in American English?

I'm having issues pronouncing barter, order, harder, smarter using General American pronunciation. I can pronounce the individual sounds as well as combinations ɾɚ as in better and ɹɚ as in bearer but the sequence of the three sounds trips me up. I think the problem is that the ɹ sound puts my tongue in a position that is so high and back that I cannot produce the flap anymore, so I force it (and touch the roof of my mouth) and then it sounds like ɹdɚ. What am I doing wrong?

Don't flap it, then — use a /t/ and not a flap; /ɾ/ and /t/ are allophones in American English, so it doesn't matter which you one use (except maybe if you're trying to speak with a perfect American accent).

Possibly the Americans who use a flap in the sequence /ɹɾɚ/ use a different variety of /r/ than you're using. I'm American, and I use one kind of /r/ in door and harder and a different /r/ in heir and weirder, and I would use a flap in door to and harder but not in heir to and weirder.