UDP has one good feature - it is connectionless. But it has many bad features - packets can be lost, arrive multiple times, there is no packet sequence - packet 2 can arrive faster than 1. How to keep good and remove bad?. Is there any good implementations that provide reliable transport protocol on top of udp so that we are still conectionless but without mentioned problems. One example of what can be done with it is mosh.
What you describe as bad isn't really bad depending on the context. For example UDP is used a lot in realtime streaming, delivery confirmation and resending is useless in this context.
That being said there are e few implementations that you might want to look at:
ENet (http://enet.bespin.org/)
RUDP (https://en.wikipedia.org/wiki/Reliable_User_Datagram_Protocol)
UDT (https://en.wikipedia.org/wiki/UDP-based_Data_Transfer_Protocol)
I work in embedded context:
CoAP (https://en.wikipedia.org/wiki/Constrained_Application_Protocol) also implements a lot of these features, so its worth a look.
What is your reason for not choosing TCP?