It's better to work on the "th" sound, as it seems to be giving you extra trouble as well. If you pronounce (<--spelling!) "repeat that" as "re-pea-tat" then the listener will immediately hear that your pronunciation is quite poor.
Hold the "t" in "repeat" - don't release it - and let it begin "that". This way will be the most natural way of saying the phrase fluently.
In general, if you have pronunciation difficulties, work on them slowly until you get them right.