I wouldn't want to guess I would just want to use a pair of vernier calipers and measure things and confirm before I start buying things. If you have to hammer in a seatpost that is usually a good sign of it is being the wrong size but I would loosen everything and measure it and always do that first and take a couple measurements around the circumference and make sure you are getting similar results.

I might consider taking it to your local shop and letting them help out as they can hone out the old seat tube and take measurements and hopefully get the correct seatpost.

Bike parts rarely if ever need to be forced and if you feel like forcing it that is a good sign it is wrong and you need to stop immediately and figure out the issue without any force. If you feel like all you can do is force it take it to a professional mechanic and let them tackle it.