This is one of those passages you gloss over because you don't want to deal with the seemingly raw conditionality of Jesus' statement. If you forgive others, your heavenly Father will forgive you; but if you don't forgive others, neither will your heavenly Father forgive you. Jesus is really tightening the screws on his earlier statement in the Lord's Prayer which was "forgive us our debts as we forgive our debtors." It's one thing to say, "to the extent that I forgive, may I be forgiven." But it's quite another to notch that idea up to: "if I don't forgive, then I won't be forgiven by my Father."
The main stumbler here is that Jesus seems to make the gospel sound suddenly conditional when we thought it was supposed to be free. Isn't the gospel supposed to be "freely you have received, freely give" and "whoever calls upon the name of the Lord will be saved"? The gospel never says, "if you believe in the Lord Jesus and forgive everyone in your life who has ever sinned against you, you will be saved."
And in fact, it would be impossible for us to forgive others as an initial condition of receiving God's forgiveness. That's because an ungenerate person is spiritually dead and incapable of that kind of grace. We love because God first loved us. Our ability to forgive flows from knowing his forgiveness. So even aside from the theological problem of making the gospel conditioned on granting forgiveness, it's a problem of logistics. Forgiveness can't be granted to others unless divine forgiveness has first been received.
For that reason I view this teaching as addressing--I guess you might say--a moral principle that governs my ongoing relationship with my heavenly Father. Jesus is teaching us how to address the Father in prayer, so he is assuming that you already have received the love and forgiveness and sonship that comes with being able to call God your Father. But when you come to him and commune with him and ask him to do stuff for you, does it make sense to be demanding forgiveness from him when you aren't willing to grant it to others? Forget about all the panicky questions that are rising up in your mind like: "How can Jesus say this?" "How can he expect me to forgive like that?" and "How am I supposed to live up to these conditions?" Just view it purely from a childlike perspective of what makes sense morally. Does it make sense to ask God's forgiveness for your offenses against him when you aren't willing to forgive other people's offenses against your piddly little self? Well . . . no, right? Logically speaking and morally speaking it makes no sense at all.
Oddly enough, I think yielding to that plain and simple truth is really half the battle. The other questions about how such forgiveness can be granted and whether you can pull it off become less daunting. They will work themselves out over time. And you can't discount the fact that forgiving others makes God's forgiveness more real to you, which in turn gives you the strength and motivation to forgive others. It's a cycle of grace that continually feeds itself.