There may be a couple of things going on if you're losing your hardon when you put on a condom.
If the condom is too snug for your length or for your girth (or both), you're probably going to find larger condom sizes to be much more comfortable and a lot less constricting. Try out a bunch of different brands and see how they feel! Some guys lose their erections regardless of condom size when putting it on or while having sex. If this sounds familiar, try using condoms when you masturbate. Eroticize them and make them a part of your sexual routine. It's easy to forget about condoms or for them to feel out of place in your sex life, so sometimes it's good to practice (and sure makes clean-up easier)! Cock rings can also help with keeping hard.
If you continue to have problems, it might be time to talk to your health care provider. Drugs are available if you are diagnosed with erectile dysfunction, just be sure to use them as prescribed by your doc (and avoid using recreational drugs like poppers at the same time, as they can have really unfortunate interactions).