You are having trouble, as many aikidoka do, becuase you are not used to sparring in this way.
Sparring is not fighting.
I suggest you learn to spar. The best way to learn to spar is....to spar.
Also, you haven't lost to your friend. What have you lost? So, you were dominated. Welcome to the world of sparring. One day you'll be totally dominating your partner, the next he'll be knocking you on the floor. That's the game.
In sparring, your partner can afford to use attacks that effect you, but don't elminate your ability to counter and/or continue. Many aikido techniques are designed for heavily commited attacks (I imagine that eventually any kind of commitment will do), which he did not need to use and which are not charaterized in wing chun training.
To tussle with strikers, I believe an understading of striking is an invaluable tool. My personal favorite combination for a wing chun striker is a cross feignt followed by a lead hook.
Sparring is a game. If you want to play, you must learn some of it's rules, as it will not conform to your ideals. Ever wonder why, no matter what style someone comes from, professional stand up fighters all look like kickboxers of some sort? It's because stand up fighting inevitable leads to certain situations that have certain efficient solutions. You aikido must be a quick, decisive, and efficient as a jab to your nose.