Don't worry this isn't just you, Lucario's biggest weakness is arguably his approach game. He has very few options to approach but once he does successfully land a hit, a skilled lucario can take it to the house.
Some tips for improving your neutral that I have picked up from the board/ matches/ experience:
1) Use more aura spheres in neutral. Aura spheres at first seem almost useless because of how slow they are but they actually are one of his more powerful options imo. The key is to place them at a height where they will limit options to your opponent. If you are being approached from the air a lot (i.e. playing against Falcon) put one at short hop height/ full hop height. Aura spheres usually get respected due to their power and then it's all up to what your opponent does. If they try to hit the sphere out of the air, follow-up strong and time the approach to land just after they throw out their aerial. If they go into shield try a quick DA + fp to start a combo. Lucario is a pretty big read-based character which brings me to #2...
2) Make them come to you. Just because Luc has a sick offense doesn't mean you have to be the one to approach. Dash dance around a bit, try to bait out an approach and punish accordingly. Fake out your opponent a bit by faking an approach then backing off. Throw out a non-committal hitbox if you want (fair, jab1, ftilt) and see how your opponent reacts. Get them to chase you then turn around and punish accordingly.
3) If you want some good approach options that I use to start my strings then short hop fair is a good one. Also, MU permitting CC dtilt is a beautiful combo starter once you learn the sweetspot vs sourspot trajectories. My most reliable one (though it's less effective the more times you use it) is to dash towards them, short hop + DT through them. Usually you will be able to bait out a jab or tilt which you will DT through, then when they are stuck in the endlag for that move, turn around and quickly jab/ tilt. If you have a charge you can also SH + DT cancel + down smash + followup (ex: ASC + dash attack).