I know exactly what you are going through. We rehomed a springer spaniel 10 months ago aged 7 (female) last summer and our cocker Jake was very jealous. Anything kicked off fights, jealousy over us, toys, balls, food. They had some really nasty fights in the first 4 months both incurring injuries. In the last 6 months, however, they have had only 2. When they do fight I separate them and ignore them totally say for a few hours upwards depending on the severity of the fight. This gives them time to cool off and also possibly they realise I am unhappy with their behaviour.