As someone who took an extended break from cycling I can say this.
I feel that you have gotten back on the bike and are trying to do the same intensity of riding you were doing before your break. Your body is trying to tell you to back off a bit. I can relate to this. It is frustrating to ride up a hill that I considered very easy for myself then find that I am struggling, looking for lower gears to make it to the top, gasping for air. It is going to take several months to get back to that top shape you had. The simple answer is that you aren't as fit as you were in September. What this does teach you, is that you were very fit before. Sometimes it's hard to judge how fit we are until we take a break, then we realise pretty quickly how fit we must have been to storm over those hills.

It will take some time, but keep at it.

CHEERS.

Mark.