Your issue isn't entirely clear, but from what I can glean, practicing tongue twisters is your best bet. Tongue twisters repeat tricky sounds in combination, and mastering tongue twisters (being able to say it reliably all the way through correctly) should help you over this problem.
Here are a couple of deceptively simple ones (say each one 3 times fast):
- rubber baby buggy bumpers
- toy boat
- flash fried flounder
You can find plenty more online. A couple of classic long ones (no need to repeat, just get all the way through) are "peter piper", "she sells sea shells", and "how much wood could a woodchuck chuck".
Of course, since you have a specific problem, you should look for a tongue twister that puts those particular sounds together. Or just make up your own. The point is to practice. This is a question of training your muscles to do what you need.
Also, being a non-native speaker, you may find that the tongue twisters that are hard for native speakers are no problem for you, and vice versa.