It's not just that replying back these 3 words is really hard, these 3 words whenever my mom, sister and my best friend, whose love I totally don't deserve, say, I cannot bring myself to say the same words or --the worse-- I just try to reply with a shitty hesitancy "yeah me too -- I guess--" All I wanna be able to is to say these words back or to show at least %50 of the love they show me that they deserve in return. But I will never be able to say these words back, why does it hurt to say these 3 words to someone who really deserves to hear them.
I mean, i'm not gonna pretend like i understand what this feels like, i can say these words to family members no problem, my biggest issue is feeling like it isn't genuine, but i can say it just fine.
For you to not even be able to say these 3 words in succession, well, i don't know what to say, with the way you talk about these 3, it seems like you really do love them, so saying that shouldn't be a problem.
The biggest problem here seems to be this feeling of not deserving that love, and to that i say, as harsh as it may seem, it doesn't matter if you feel like you deserve people's love or not, they still love you, it's still real, cuz it's something that comes from them, it's under their control above all else, not yours.
What is under your control tho, is what you feel towards them, and i can see that it's love, so own it, say it loud and clear.
Now, i know it's probably more complicated than that, so we need more motivation, i'll try to give a few tips to help you succeed in this:
1. Try hugging these people, it's much easier to say things like "I Love You" in a moment of affection, like while hugging.
2. Practice, if you have pictures of them on your phone, look at these pictures and say it, so you'll become more used to it.
3. Talk about these insecurities of yours with them, they'll probably say something similar to what i've said, that they love you regardless.
Again on "3", being open about your feelings to people you clearly love, can be helpful in overcoming any mental illness you might have, like depression, i assume.