I appreciate your openness. Kudos. I think fat tissue is a fair proportion of breast mass so working out that area will hit the size of them. When you stop training they'll likely resume to how they were. Being realistic, how many big-bosomed and muscular women do you see around; not surgically augmented? If you want the muscular physique I think it goes with the territory/activity that your boobs will get smaller; look at the Olympic swimmers and, in fact, any seriously competitive female athlete:
Swimmer Alicia Coutts is pretty typical
I honestly don't think, for the majority of guys that they are an arbiter of whether they fancy you or not; not ones worth knowing anyway. I think you may be experiencing confirmation bias.
Example: you are likely thinking: "He likes her because she's got big boobs". I bet you are also subconsciously ignoring (confirmation bias) all the other guys with girls that have less there because you want to confirm what you think, that small-breasted women don't get the guys; making you feel inadequate. Fact: Not all men prefer big boobs; This is besides the fact that I don't think they rate terribly highly on any worthwhile-man's list of priorities.
I honestly think your current lack of self-confidence is just from a lack of experience dating. It doesn't matter how old you are it's a nerve-wracking exercise the first time. Take it one step at a time; just talk to the guys as people and don't look at them as potential partners. That way you aren't putting pressure on yourself and it will happen if you relax and just be yourself. In time, with practice just generally socialising, you'll eventually feel more confident to be assertive towards those you do want to home in on.
Back to your OP: given that you are naturally sufficiently endowed breast-wise normally, I would forget about any kind of enhancement because they will come back when you move on from your current lifestyle/activitity to a more sedate one.
Enjoy being as you are now; fitter and more athletic. The boobs can wait; you know they are already there.
You can't be all things to all people; stop trying and just focus on fitting into the social niche you feel comfortable in.
I hope this, and other contributions here, helps put things in a truer perspective for you.