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If your old fuel was E5, then it will probably have gone off after three months so, after over four, you'll definitely have problems. The ethanol absorbs moisture from the air and, when it becomes saturated and can't absorb any more, it comes out of suspension, separates from the petrol, and you have a horrible cocktail of water and ethanol sitting at the bottom of your tank. How quickly this happens depends on how humid your garage is - the more moisture that's in the air, the quicker the ethanol will separate.
If your old fuel was ethanol-free, then it will still go off over time, but should last about six months. It won't absorb water but the lighter chemicals in the fuel, the 'volatiles' will evaporate leaving a less combustible mixture behind. Also, the fuel reacts with the oxygen in the air and oxidises, which is not good for combustion. (If the fuel smells sour or has turned dark, then it's probably oxidised.)
Either way, I wouldn't try to use up the old fuel if I were you. You'd be better off draining the system completely and refilling with an ethanol-free brand like Esso Synergy Supreme+. The hesitancy you are experiencing is probably a combination of varnish in the fuel system components, water in the fuel and the fuel itself going off.
You may find running a couple of tanks of fresh fuel through the system cures the problem or you may have to add a cleaning additive. If that doesn't work, then you may have to look at removal and deep cleaning the carbs.
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