I can't be sure what your problem is, but here are a few suggestions.
Be sure the condom fits. Most condoms are one size fits most - but some are designed for smaller or bigger guys. Be sure you haven't bought a kind for smaller guys by mistake.
The other thing is to put a dab of lubrication on the end of your penis and then it will slide on more easily.
Practice using a condom when you masturbate. If you use a condom more often if will be less of a big deal when you put it on when somebody else is around.
Good luck. Let me know if any of these ideas help!
Susan