It sounds to me as though you love deeply and genuinely. I'm struggling to believe the two people you mention are as genuine if, as you say, they were declaring love right up until you broke up.
Were there any other signs, with hindsight, that perhaps they weren't always deeply honest? Maybe little inconsistencies between what they did and what they said?
I wonder because if they can declare love and then very quickly end the relationship that is a fundamental mismatch between words and deeds. From where I'm standing the new girlfriend doesn't have much of a prize that being the case.
I was with a bf before my now husband, the relatiinship had its problems and I told him I was unhappy, in fact had ended things but been persuaded to try again, so when I broke up with him it was no surprise. I then randomly met my now dh one week later and was in a relationship with him only 2 weeks after bf and I broke up. But my feelings had been dying for some time, I'd already emotionally let go of the relationship with my bf some time before so the moving on wasn't as quick as it looked. But then I wasn't telling my bf at the time I was happy with him!
I think if you can be with someone and notice if they are emotionally honest (do they say to their mates the same they say behind their back, do they do the right thing even if they think no-one is looking, are they open about it if they are unhappy so give constructive criticism kindly not leave to guessing or pretending it's all fine... etc etc), then you are in a better place to know that declarations of love are real and deep, not just being said cos it's what you want to hear.
But crucially you have to allow the other person to love you as comes naturally to them, some people can be coerced by emotional pressure to profess a love they don't truly feel. So you need to offer your love in the way that comes naturally but without requiring any particular return. If you can do that then whatever comes your way is freely given.
So if they are fundamentally true to themselves, not 2 faced in any way, and you accept their love as they freely offer it, and they love you in a way that is compatible with what you need then this situation you are suffering becomes much less likely.
I'm sorry you are hurting, and I can see why you regret sending the card, but you won't do that again. It is hard when you have a lot of love with nowhere for it to go, so be forgiving of yourself, it shows you care deeply and when you meet the right person they will be lucky to have that.
Seek the honest guys and see how they offer love without trying to influence that then you'll know where you stand.