OP: "A very common struggle newcomers have with Christianity is reconciling the terrifying God of the Old Testament with the loving God of the New Testament."
The idea of fear with respect to a perception of the terrifying threat and danger from God as negative motivation is as much a Christian idea as it is Jewish. Indeed no less than Christ himself sought to motivate His disciples to proclaim boldly the gospel by reminding them of the threat of body and soul destruction in hell.
27 What I tell you in the dark, say in the light, and what you hear whispered, proclaim on the housetops. 28 And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell. -Matthew 10:27-28
4 “I tell you, my friends, do not fear those who kill the body, and after that have nothing more that they can do. 5 But I will warn you whom to fear: fear him who, after he has killed, has authority to cast into hell. Yes, I tell you, fear him! -Luke 12:4-5
And when the Hebrew writer desired to motivate Jews to not forsake the Christian life he wrote:
26 For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. 28 Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. 29 How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? 30 For we know him who said, “Vengeance is mine; I will repay.” And again, “The Lord will judge his people.” 31 It is a fearful thing to fall into the hands of the living God. -Hebrews 10:26-31
We also have the example of Paul who tells us to work out our salvation in fear and trembling (Phil 2:12, Eph 6:5).
12 Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling, 13 for it is God who works in you, both to will and to work for his good pleasure. -Philippians 2:12-13
Ephesians 6:5 5 Bondservants, obey your earthly masters with fear and trembling, with a sincere heart, as you would Christ,
Indeed, even Paul was careful being motivated by the dreadful possibility that he, himself, would be disqualified even after all he's done.
27 But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified. -1 Corinthians 9:27
How do we reconcile 1 John 4:18 (along with Rom 8:15 & 13:3)? I think there's a difference between being aware of the fearful consequences of not living a Christian life and expecting those dreadful consequences. Christians should always keep a humble, fearful, awareness of the consequences of leaving the way of Christ, but we should also feel secure that as we continue to follow Christ, following the narrow path, we will be preserved on that day.
With regard to Jesus being a liar.
There is no such danger.
In Luke 11:12-13 it reads:
12 or if he asks for an egg, will give him a scorpion? 13 If you then, who are evil, know how to give good gifts to your children, how much more will the heavenly Father give the Holy Spirit to those who ask him!”
The context is that if you ask for the Holy Spirit from the Father, you'll get the Holy Spirit from the Father. It says nothing about whether we should fear God.
Ultimately, I believe that Christians should continue to see fear as educational and motivating. We should not live in terror, unless you realize you are not following Christ, then terror is quite rational, especially immediately followed by repentance. Barring that, we should always have a healthy awareness of the consequences of God so that we are careful to live according to Christ.