Help Pronouncing τρ

I am going through Dr. BPK's Koine Greek introductory course with the Biblical Language Center. It is very difficult pronouncing some of the new sounds! I am OK at pronouncing ρ as a "tap" or even as a "trill," but it is VERY difficult for me when it follows τ, as in μαθήτρια. Does anyone have any tips for pronouncing this? Sometimes it just feels like my mouth is falling apart into a weird "L" sound. Sometimes I can't make the sound at all if my mouth is tired from the exercises. At this point, I might be able to "force" it by making the sound louder or sharper (putting more force into the aspiration).

Also - bonus if you can help me pronounce υ / οι. I've tried making the ι sound and then just bringing my lips together, especially focusing on puckering-up my bottom lip.