3 ways to tell your past feeling

To describe when you thinks that something had been bound to be happen, however it turned out to be different. For example, I wasn't supposed to be going to the party, but the manager couldn't make it, so I had to go after all.
パーティーは行かないはずでしたが、部長は都合が悪いことになっていたので、行くことになりました。

To describe when you had intended to something. For example, I had't intended to go to the party, but it looked like it would be fun, so I decided to go after all.
パーティーは行かないつもりでしたが、面白いそうなので行くことにしました。

To describe when you thinks that something to be X, however in reality it may not be X at all. For example

A：鍵が掛かっていませんでしたよ。
B：すみません、掛けたつもりでした。
B：そんなはずはありませんよ。