AIRIC UL Listed 200 Pcs Wire Butt Splice Connectors Kit, 22-10 AWG Electrical Crimp Terminal Connector Kits, Fully Insulated Wiring Straight Crimps Terminals for 22,20,18,16,14,12,10 Gauge Wires

In the world of electronics, soldering two wires end-to-end is frowned upon for the reasons you are experiencing. Use a mechanical connection instead, no soldering needed. I like both the heat shrink connectors and the crimped connectors. In a high heat environment like an RC car I would go with the crimped connectors, but it is best to learn how to properly crimp them first.