Normally in a debate you would think of either side of the argument. (Being able to contradict your own points).

You may be stuck with arguing a side of an argument that you do not necesarilly believe in, however this is a good learning process, and may actually teach you new ways of thinking.

In terms of you being angry at not being able to do topics that you like more, most of the time a vast amount of topics can be used to debate in a certain topic.
For example, in the case of "For Abortion", you could mention many Catholics views, that life begins at Conception, and therefore abortion for them is murder, and thus goes against the commandment of "do not kill". Also life is God given in many Catholics opinion, and therefore life is sacred, and only God can take away life. I do not personally believe these things, but I recall debating these points when I was in your situation a year or two ago. There is a huge possibility of things you can say, and each new point is an extra bit of knowledge for you.

Conclusion.
Just because you have to debate in a certain side, does not mean you have to believe what you are arguing. Trying to debate both sides of the argument, will really make you get a better idea of that topic.
Also as others have said, when debating you do not necessarily have to agree with what you are saying, it is all part of the learning process.

Goodluck.

I love this argument.
Science of course. Anyone who wants to challenge that, send me a PM.