Happy birdthday and take care what decision you make.. if you want to think about marrage remember it's the whole package you need to think about, i.e. the morals and ideals of the person, his dedication. if your boyfriend does not care about your birthday it does not mean he does not care about you. i'm just saying people need some space sometimes , and even if 2 people are married they still need some space to do their own things.
On the other hand, your ex might have bin a diffirent person and changed, men go through a lot around the 20s and many of them like to play around but at one point or another they want to settle down and have a family security, i can't tell you if he's completly honest this time , but i think you do owe him another chance, if he does it again though then that's it! make sure he understands that.
Oh btw your story is very similar to Brigite Jones the movie. i don't want to spoil in case you didn't see it yet, but if you really haven't seen it then GO SEE IT NOW, you will relate it's story EXACTLY like yours -:)