First of all, I think saying you were wrong to take that long a vacation is a really sad conclusion to land at. That to me would be the parenting equivalent of saying it isn't worth finding love because heartbreak is too painful. It's inconceivable to me that you clould harm your child by spending too much time with them, being too close or too loving. What you're going through now may be painful for you both, but it is testament to the fact that spending time with you is still best, and I would be more concerned if the opposite was the case.
Having said that, I think you can help your child by droping them off in a playful manner. This example is going to be overly specific, but just to give you an idea:
In the hallway, say "Ok, bye, I'm leaving for work", and then you playfully walk out the wrong door, into the toilet or something. Or say "OK, you go off to work now, and I'll stay here and play all day long until you come and pick me up, bye bye". I wouldn't be surprised if your child will laughingly show you the door.
Fool around for a bit, and when you switch to actually leaving, you can in a light-hearted manner say "bye", and then "oh no, I'm not ready to go yet, I need more kisses," and then you play-leave for a few rounds before actually leaving.
Your objective here is three-fold:
- To validate your child's separation anxiety. Show that it is OK to feel sad that you'll be away from another, and that you feel that too.
- Importantly, model a playful rather than tearful outlet for that feeling.
- Lastly, by doing this you help the child transition into play. It shouldn't be a harsh disconnect from the comfort of being with you. Here's there to play, and it's understandable if he can't make that switch on command. This way you'll get him started on playing, and daycare may not seem as threatening.
Of course if you can pick him up earlier I think that can't hurt either, but once he gets stated playing, I'm not sure it'll be needed.
Don't stop taking long vacations.