Bottoming for the first time

I have just recently came out as gay (more so bisexual) because I have met this really awesome guy. We have tried having sex already but it failed. Let me take you back. We have known each other for a little over a month now. We have been officially together for 2 weeks. Within that time we have slept with each other 3 times. The first time we did nothing except sleep. Boring I know. But I'm still a virigin so wanna take it slow. The second time we only did what you do in foreplay. Kissing, handjob, blowjob etc...he came but I didn't cum as the sensation went away after a while and I went soft. He said he didn't mind as he knew that this was my first time with a guy. The third time which was today I was planning to go all the way with him and had prepared everything. We started off with foreplay, kissing licking blah blah and his boxers were drenched with precum. We got to the point where we were fully naked and he was ready to cum. I still felt a far way from that. He came and he said that he wanted to go another round. I was okay with that because I still had not yet came. Throughout the time it felt as if I was losing the feeling and my penis would get soft and hard again. It would feel as if I was about to finish and then the feeling would just go. I asked him if he would like to take my virginity and he said yes. So I got the condom for him and we tried to get him to penetrate but it just wouldn't go in. After about 5 minutes I got soft even after trying to get hard again and he said he didn't want to finish this round. I'm not sure what I'm doing wrong. Why do I keep going soft and not finishing? Why can't he penetrate me? I feel he might lose interest if I can't finish when I'm with him. Any advice please....