First of all, it sounds like you're completely normal with the too-quick problem. Most all young men, at least for the first 5 or 6 years of being sexually active, have a problem lasting. It gets better with practice
. Best advice is to just do it more often, and get to know where your point of no return is. When you feel that getting close, just STOP for a minute, and you will reset your timer and can keep going. If you're with a really hot girl, you're just going to explode, and that's a compliment to her. At least at <30, you can probably let one blow right away, then do something for her, then you're ready again in 10 minutes... (I wish I could trade my over-40 performance problems for your under-30 speed issues)....
On the problem of going soft after you're in, that could most likely be a mind-over-matter thing. Best thing is to talk to a professional; a counselor, therapist, etc., and see if there's something bugging you about the more recent relationships. Could be that subconsciously, you feel something isn't right about these women for you, or you're still partly in love with your ex, or maybe there are other simple things in your life that are on your mind and you don't recognize it as being part of this problem. It's very doubtful that you have anything physically wrong, as long as your blood pressure is OK, and you don't have any underlying physical problems (diabetes, obesity, etc etc) that would affect this.
Charles