How does one discern between thoughts inspired by the Holy Spirit and those produced from themselves This question has been nagging at me for a while, how can one tell the difference between honest-to-God inspiration from the Holy Spirit and thoughts that are induced by themselves out of the needs or fears of the person.  The Spirit is described as a 'still small voice' which can be strikingly similar to the things you tell yourself if you want to convince yourself of something.

Example: I have this friend who went on about three dates with a guy before he proposed to her claiming he had prayed to God and received revelation from the Spirit that they were meant to be together. She of course didn't buy into it and rejected him flat out, because he had convinced himself of revelation that was simply untrue, no matter much he wanted it to be.

How can we prevent ourselves from making similar mistakes?
The Catholic Church has a fairly well-established attitude (and rules) towards private revelation. They're summarized in Contemplation - Revelations and visions. The first two, most important, "rules" regarding private revelation are explicitly mentioned:

Firstly, the spiritual director must judge whether the revelation directs the person to deeper love of God or the saints. If so, the revelation is at least "provisionally" inspired. If not, and especially if a call to some [costly] endeavor is present, the director must demand signs from God. (Note: The first rule implies that the person has a spiritual director, and that the spiritual director knows their prayer life!)

Secondly, the receiver of a revelation should resist the revelation, accepting it only after a spiritual director decides that the "seer" should place some confidence in them. (Again, an experienced spiritual director is implied!)

These particular rules seem pretty strict, intended to safeguard the "seer" and The Church against "big", public deceptions. But, slightly less stringent rules along a similar vein apply to revelations of a seemingly limited, personal matter. In particular, I'd suggest looking into St. Ignatius's Discernment of the Spirits. On the more strict end, there are 22 rules. But, the gist of it is knowing and loving God, first and foremost, and then recognizing spiritual "consolation" and "desolation."

In very rough terms, consolation is a feeling of "being set on fire with love for God." Desolation is a feeling of uncertainty, turmoil, or confusion. The basics of discernment involve, usually under the guidance of a spiritual director, imagining a decision and feeling whether it gives you consolation -- whether it sets you on fire with love for God. If not -- and especially if desolating -- it's probably not God's call.

It might be worth picking up a copy of Ignatius's spiritual exercises.

I think you'll find that the rules for discernment also call for a spiritual director. And that they also call for some reluctance in leaping to assume you've heard God's voice.

Remember: If God wants you to know something, He's not going stop bugging you after the first call. The prophets had a tendency to ignore their visions at first. Be a good prophet and do the same!