Don't be afraid to record yourself and ask for honest feedback. Some members here use vocaroo.com or clyp.it (no account needed) and then they post the link. Only about 30 seconds to 1 minute of speaking is enough. Talk more with people who have the accent you'd like. The truth is, you'll get your accent from the people you talk to.

A speech therapist usually works with native speakers. A (good) ESL (English as a second language) teacher, on the other hand, uses similar techniques, but has specialized knowledge and skills for helping non-native speakers.

Good suggestion from Mr Peachey. A recording should really help us get a better idea of what's going on with your pronunciation. By the way, I know you said not to ask how you pronounce “archive,” but actually, I’m quite curious, because it doesn’t seem like it should present major pronunciation problems. English versus Polish: English is heavily stress-timed, whereas Polish lacks phonemic stress entirely and is very syllable-timed. This difference in isochrony does not, however, explain your difficulty with “archive.” Also, Polish has lots of consonant clusters in initial position, while English has lots of consonant clusters in final position. Again, this doesn’t explain your difficulty with “archive” either. In theory, “archive” is quite easy to pronounce, but the spelling is a bit confusing (the CH is pronounced K), and there may be interference from your native language since you have a similar word. (I’m guessing that you want to pronounce the CH in a way that sounds like an H to English speakers.) Here is how I’d spell the English word “archive” phonetically in Polish: arkajw or non-rhotic: aakajw